How to Pray Witr If You Combine Maghrib with Isha
Question

What is the ruling on praying Witr at the time of `Isha’ when travelling and shortening the obligatory prayers?
Praise be to Allah.Shortening prayers while travelling 
“Travelling is a reason which permits the four-Rak`ah prayers to be shortened to two Rak`ahs , rather it – i.e. travel – is the reason which dictates shortening four Rak`ah prayers to two Rak`ahs , and this is either obligatory or recommended, depending on the differences in scholarly opinion. 
The correct view is that shortening prayers is recommended and is not obligatory; although there are some texts which apparently indicate that it is obligatory, there are other texts which indicate that it is not obligatory.” 
The four-Rak`ah prayers are Dhuhr, `Asr and `Isha, and the evidence for that is the Book of Allah and the Sunnah of His Messenger (peace and blessings of Allah be upon him), and the consensus of the ummah. 
In the Quran, Allah says (interpretation of the meaning): 
“And when you (Muslims) travel in the land, there is no sin on you if you shorten the prayer if you fear that the disbelievers may put you in trial (attack you).” [An-Nisa’ 4:101]
And evidence is to be found in the actions of the Prophet (peace and blessings of Allah be upon him): “When the Prophet (peace and blessings of Allah be upon him) travelled, he would pray two Rak`ahs. It was not narrated from him that he (peace and blessings of Allah be upon him) ever prayed four Rak`ahs whilst travelling, rather during all of his journeys, long and short, he would pray two Rak`ahs .” 
With regard to the consensus of the Muslims, this is something which is a well-known and well-established part of the religion, as Ibn `Umar (may Allah be pleased with him) said: “I prayed behind the Prophet (peace and blessings of Allah be upon him) and Abu Bakr, ‘Umar and `Uthmaan, and they never prayed more than two Rak`ahs when travelling.” And the Muslims are agreed upon that. 
Praying Witr when combining ‘Isha with Maghrib 
The traveller may pray Witr after praying `Isha with Maghrib and praying them at the time of Maghrib.” (See Fataawa al-Shaykh Ibn ‘Uthaymeen, vol. 1, p. 412; al-Sharh al-Mumti’, vol. 4, p. 502; Fataawa al-Lajnah al-Daa’imah, vol. 8, p. 144)
For more details about Witr, please refer to these answers: 46544 , 36793 , 32577 , 216236 , and 65692 .
And Allah knows best.