Is It Sunnah to Read Al-Mulk and As-Sajdah between Maghrib and `Isha’?
Question

Is there any report which speaks about reading Surat As-Sajdah and Surat Al-Mulk between Maghrib and `Isha’? or about reciting three verses of Surat Al-An`am immediately after Fajr prayer?
Praise be to Allah.Degrees of Hadiths about virtues of the Quran
Before replying to this question, we must establish an important point about the virtues of certain Surahs.
There are fabricated Hadiths about the virtues of various Surahs which have been falsely attributed to the Messenger of Allah (peace and blessings of Allah be upon him). Among the most famous of those who are known for that are the following:

Nuh ibn Abu Maryam Al-Jami`, of whom it was said: He encompassed everything except the truth. He regarded it as permissible to tell lies in Hadith in the interests of the religion, and he made up Hadiths by himself and attributed them to the Messenger (peace and blessings of Allah be upon him) concerning the virtues of the Surahs of the Quran, Surah by Surah.

Abu `Ammar Al-Husayn ibn Hurayth Al-Marwazi said: It was said to Abu `Ismah – i.e., Nooh ibn Abi Maryam – : “From where do you get (a Hadith) from `Ikrimah from Ibn `Abbas (may Allah be pleased with him) concerning the virtues of the Qur’an Surah by Surah, when the companions of `Ikrimah have no such Hadith?” He said: “I saw that the people had turned away from the Qur’an and were distracted by the fiqh of Abu Haneefah and the Maghazi of Muhammad ibn Is-haq, so I made up this Hadith, seeking reward.” (Narrated by Al-Hakim in Al-Madkhal (p. 54); Ibn Al-Jawzi in Al-Mawdu`at (16). Its chain of narration is authentic)

Maysarah ibn `Abd Rabbihi Al-Farisi, of whom Ibn Hibban said in Al-Majruhin (2/345, no. 1038): He is the author of a lengthy Hadith about the virtues of the Quran, in which it says: “Whoever recites such and such will have such and such.”

In Lisan Al-Mizan (7/198) by Al-Hafidh ibn Hajar (may Allah have mercy on him) it says: “In Ad-Du`afa’, Ibn Hibban narrated from Ibn Mahdi that he said: I said to Maysarah ibn `Abd Rabbihi: “From where did you get these Hadiths, `Whoever recites such and such will have such and such’?” He said: “I made them up in order to encourage the people.”
These are examples of people who made up false Hadiths and attributed them to the Messenger (peace and blessings of Allah be upon him) for a purpose that they thought fit, by means of which Iblees deceived them.
The scholars drew attention to the fact that the Hadiths which list the virtues of all the Surahs of the Quran Surah by Surah, are not proven. Among those who drew attention to that was Al-Mawsili in Al-Mughni `an Al-Hifdh wal Kitab (1/121). He said: And he narrated: “Whoever recites such and such will have such and such… from the beginning of the Qur’an to the end. Ibn Al-Mubarak said: I think this was fabricated by the zindeeqs (heretics). The editor – i.e., Al-Mawsili – said: There is no authentic report concerning this.
Attention was also drawn to this by Ibn Al-Qayyim (may Allah have mercy on him) in Al-Manar Al-Munif (p. 113-144) and Shaykh Bakr Abu Zayd in At-Tahdith bima qila: “There is no authentic report concerning this.” (p. 122-123). He added: It should be noted that the virtues of the Holy Quran and the virtues of certain Surahs and verses are known from authentic texts which are attributed to the Prophet (peace and blessings of Allah be upon him). What Ibn Al-Mubarak and those who came after him meant was to discuss the lengthy Hadiths which discuss the Surahs of the Quran Surah by Surah, such as the Hadith which is attributed to Ubayy ibn Ka`b (may Allah be pleased with him), which was quoted by some of the Mufassirin – such as At-Tha`labi, Al-Wahidi and Az-Zamakhshari – in their Tafseers, which is fabricated.
This is what is meant by the discussions of Ibn Al-Mubarak and others. And Allah knows best. (End quote)
Is It Sunnah to Read Al-Mulk and As-Sajdah between Maghrib and `Isha’?
It was narrated that `Abdallah ibn `Umar (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever reads {Alif-Lam-Mim. The revelation of the Book} [As-Sajdah 32; interpretation of the meaning] and {Blessed be He in Whose Hand is the dominion} [Al-Mulk 67; interpretation of the meaning], between Maghrib and `Isha’, it is as if he spent Laylat Al-Qadr in prayer.”
This was quoted by Al-Suyuti (may Allah have mercy on him) in Ad-Durr Al-Manthur (6/535) at the beginning of Surat As-Sajdah. He said: “Ibn Mardawayh narrated from Ibn `Umar …” and quoted it.
Al-Alusi (may Allah have mercy on him) quoted it in Ruh Al-Ma`ani (21/116) from Al-Suyuti, then he said: “A similar report was narrated by him – meaning As-Suyuti – and Al-Wahidi from the Hadith of Ubayy ibn Ka`b, and by At-Tha’labi from the Hadith of Ibn `Abbas. Wali Al-Din commented on that by saying: “I do not agree with that; all of these reports are fabricated.” (End quote)
Several versions of the Hadith have been narrated, some of which are general in meaning and do not specify a time for reading, and some of which do specify a time, as in the report of Ibn `Umar. It has been narrated as Marfu` (attributed to the Prophet)` and Mawquf (attributed to a Companion). Al-Ghafiqi (may Allah have mercy on him) mentioned them in Lamahat Al-Anwar (1127, 1129, 1140, 1141, 1142, 1143, 1144, 1146), except the report of Ibn `Umar.
Any virtues of Surat Al-An`am
It was narrated via two chains of transmission:

From Ibn `Abbas (may Allah be pleased with him) in a Marfu` report: “Whoever recites when he prays Fajr three verses from the beginning of Surat Al-An`am up to the words (interpretation of the meaning): {and He knows what you earn (good or bad)} [Al-An`am 6:3], forty thousand angels will come down to him and the like of their deeds will be written for him. And there will be sent to him an angel from seven heavens with an iron rod, and if the Shaytan instills any evil in his heart he will strike him until there will be seventy veils between him and him. When the Day of Resurrection comes, Allah will say: `I am your Lord and you are My slave, walk in My shade, and drink from Al-Kawthar, and wash in As-Salsabil, and enter Paradise without being brought to account or punished.’”

This was mentioned by As-Suyuti (may Allah have mercy on him) in Ad-Durr Al-Manthur (3/245-246). He said: As-Sulafi narrated it with an unreliable chain of narration from Ibn `Abbas as a Marfu` report, and Al-Ghafiqi mentioned it in Lamahat Al-Anwar (941).

It was narrated that Ibn Mas`ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever prays Fajr in congregation and sits in his prayer place, and recites three verses from the beginning of Surat Al-An`am, Allah will appoint seventy angels for him who will glorify Allah and pray for forgiveness for him until the Day of Resurrection.”

This was quoted by As-Suyuti (may Allah have mercy on him) in Ad-Durr Al-Manthur (3/246) and he attributed it to Ad-Daylami and Al-Ghafiqi in Lamahat Al-Anwar (935) in a version close to that of the Hadith of Ibn `Abbas.
Al-Alusi (may Allah have mercy on him) said in Ruh Al-Ma`ani (7/76), after quoting a number of Hadiths and reports about Surat Al-An`am, including the Hadith of Ibn `Abbas and Ibn Mas`ud (may Allah be pleased with them): “And there are other reports, but most of that is da’eef (weak) and some of it is mawdu` (fabricated), as is obvious. (End quote)
There is no proven Hadith which speaks of the virtues of Surat Al-An`am.
With regard to Surat As-Sajdah and Al-Mulk, there is no proven report about reading them between Maghrib and `Isha’, but there is a proven report about the virtue of reciting Surat As-Sajdah in Fajr prayer on Friday.
Al-Bukhari (891) and Muslim (880) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) used to recite in Fajr prayer on Friday: {Alif-Lam-Mim. The revelation…} [As-Sajdah 32; interpretation of the meaning] and {Has there not been over man a period of time…} [Al-Insan 76; interpretation of the meaning]
And concerning the virtue of Surat Al-Mulk, it is narrated that it may be recited when going to sleep or in general. At-Tirmidhi (2891), Abu Dawud (1400) and Ibn Majah (3786) narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: There is a Surah in the Quran, with thirty verses, which will intercede for its companion [the one who recites it] until he is forgiven: “Tabarak alladhi bi yadihi’l-mulk {Blessed be He in Whose Hand is the dominion} [Al-Mulk]. At-Tirmidhi said: This is a sound Hadith.
Ibn Hajar (may Allah have mercy on him) said in At-Talkhis (1/234): “Al-Bukhari stated that there is a problem with it in At-Tarikh Al-Kabir by noting that it is not known that `Abbas Al-Jashami (who is the one who narrated it from Abu Hurayrah) heard it from Abu Hurayrah.” (End quote)
It was classed as sound by Al-Albani in some places and as authentic in others. (Sahih Sunan Ibn Majah, Sahih Sunan Abu Dawud). Before him Al-Mundhiri (may Allah have mercy on him) said:  It was narrated by Abu Dawud, by At-Tirmidhi, who classed it as sound and this version was narrated by him, and by Al-Nasa’i, Ibn Majah, Ibn Hibban in his Sahih, and by Al-Hakim who said its chain of narration is authentic.
At-Tirmidhi (2892) narrated from Jabir (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) did not sleep until he had recited {Alif-Lam-Mim. The revelation…} [As-Sajdah 32; interpretation of the meaning] and {Blessed is He in whose hand is dominion…} [Al-Mulk 67; interpretation of the meaning]  (Classed as authentic by Al-Albani in Sahih At-Tirmidhi)
For more details, please see this category: Virtues of the Quran.
And Allah knows best.