How Many Religions Believe In The Trinity?

Christianity is the only religion that believes that stuff, though there is something similar in some ancient no longer practiced religions.

The three monotheistic religions - Judaism, Christianity, and Islam -
all purport to share one fundamental concept: Belief in God as the
Supreme Being, the Creator and Sustainer of the Universe. Known as
"tawhid" in Islam, this concept of the Oneness of God was stressed by
Moses in a Biblical passage known as the "Shema," or the Jewish creed
of faith:

"Hear, O Israel: The Lord our God is one Lord." (Deuteronomy 6:4)

It was repeated word-for-word approximately 1500 years later by Jesus when he said:

"...The first of all the commandments is, Hear, O Israel; the Lord our God is one Lord." (Mark 12:29)

Muhammad came along approximately 600 years later, bringing the same message again:

"And your God is One God: There is no God but He, ..." (The Qur'an 2:163)
Christianity has digressed from the concept of the Oneness of
God, however, into a vague and mysterious doctrine that was formulated
during the fourth century. This doctrine, which continues to be a
source of controversy both within and without the Christian religion,
is known as the Doctrine of the Trinity. Simply put, the Christian
doctrine of the Trinity states that God is the union of three divine
persons - the Father, the Son and the Holy Spirit - in one divine
being.
If that concept, put in basic terms, sounds confusing, the
flowery language in the actual text of the doctrine lends even more
mystery to the matter:
"...we worship one God in Trinity, and Trinity in Unity... For
there is one Person of the Father, another of the Son, another of the
Holy Ghost is all one... They are not three gods, but one God... The
whole three persons are co-eternal and co-equal... He therefore that
will be save must thus think of the Trinity..." (excerpts from the
Athanasian Creed)
Let's put this together in a different form: One person, God
the Father + one person, God the Son + one person, God the Holy Ghost =
one person, God the What? Is this English or is this gibberish?
It is said that Athanasius, the bishop who formulated this
doctrine, confessed that the more he wrote on the matter, the less
capable he was of clearly expressing his thoughts regarding it