what religions believe in the trinity?

Other then Christianity, a few. Wiccans believe in the the three aspects of the Goddess...Maiden, Mother, and Crone. The Hindu's believe in the Trinity of Brahma, Vishnu, and Shiva.

Or did you just want the denominations of Christianity that do? Nontrinitarism is the term that is used for the Christian denominations that do not believe in the Trinity (Father, son, Holy Spirit). They include Jehovah's Witnesses, Mormons, Seventh Day Adventists, and Unitarians (among a couple of others).

No. Jehovah's Witnesses do not. Why? With Facts? Well the scriptural proof that Jesus is God's son, separate and distinct from the father but let's start with two glaring commends by Jesus

(John 14:28) YOU heard that I said to YOU, I am going away and I am coming [back] to YOU. If YOU loved me, YOU would rejoice that I am going my way to the Father, because the Father is greater than I am.

(John 5:30) I cannot do a single thing of my own initiative; just as I hear, I judge; and the judgment that I render is righteous, because I seek, not my own will, but the will of him that sent me.

Scriptures like John 10:30 are misinterpreted to mean actual, literal oneness and not the same oneness of through conveyed in John 17:11.

Only Christianity believes in the Trinity. To believe in it, you are a Christian, no matter what the religion you call yourself.

The Trinity has become Christianity's easiest and simplest doctrine to understand now that the mystery has been reveal to us.

Catechism

Christians believe in it and I do too. I believe it because it's in the Bible.

Christianity and I believe in it because I have faith.

Just the false ones.

I believe in the TRI-UNE