Islam
The three core principles in Shia tradition: Tawhid (Oneness of God), Nabuwwa (Concept of Prophethood), Imama (Concept of Imam)

Buddhism
The Triple Bodhi (ways to understand the end of birth) are Budhu, Pasebudhu, and Mahaarahath. The Three Jewels, the three things that Buddhists take refuge in. Shinto
The Imperial Regalia of Japan of the sword, mirror, and jewel. Daoism
The Three Treasures (Chinese: 三寶; pinyin: sānbǎo; Wade–Giles: san-pao), the basic virtues in Taoism. The Three Dantians
Three Lines of a Trigram
Three Sovereigns: Heaven Fu Xi (Hand – Head – 3º Eye), Humanity Shen Nong (Unit 69), Hell Nüwa (Foot – Abdomen – Umbiculus). Hinduism
The Trimurti: Brahma the Creator, Vishnu the Preserver, and Shiva the Destroyer. The three Gunas found in Samkhya school of Hindu philosophy. The three paths to salvation in the Bhagavad Gita named Karma Yoga, Bhakti Yoga and Jnana Yoga. Zoroastrianism
The three virtues of Humata, Hukhta and Huvarshta (Good Thoughts, Good Words and Good Deeds) are a basic tenet in Zoroastrianism. Norse mythology
Three is a very significant number in Norse mythology, along with its powers 9 and 27. Prior to Ragnarök, there will be three hard winters without an intervening summer, the Fimbulwinter. Odin endured three hardships upon the World Tree in his quest for the runes: he hanged himself, wounded himself with a spear, and suffered from hunger and thirst. Bor had three sons, Odin, Vili, and Vé. Other religions
The Wiccan Rule of Three. The Triple Goddess: Maiden, Mother, Crone; the three fates. The sons of Cronus: Zeus, Poseidon, and Hades. The Slavic god Triglav has three heads.
Numeral systems
There is some evidence to suggest that early man may have used counting systems which consisted of "One, Two, Three" and thereafter "Many" to describe counting limits. Early peoples had a word to describe the quantities of one, two, and three but any quantity beyond was simply denoted as "Many". This is most likely based on the prevalence of this phenomenon among people in such disparate regions as the deep Amazon and Borneo jungles, where western civilizations explorers have historical records of their first encounters with these indigenous people. List of basic calculations
Science
The Roman numeral III stands for giant star in the Yerkes spectral classification scheme. Three is the atomic number of lithium. Three is the ASCII code of "End of Text". Three is the number of dimensions that humans can perceive. Humans perceive the universe to have three spatial dimensions, but some theories, such as string theory, suggest there are more. Three is the number of elementary fermion generations according to the Standard Model of particle physics. The triangle, a polygon with three edges and three vertices, is the most stable physical shape. For this reason it is widely utilized in construction, engineering and design. The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans being trichromatic, the retina contains three types of color receptor cells, or cones. There are three primary colors in the additive and subtractive models.