Not true. Christians believe in only one God. Indeed, that is the opening line of the Creed recited at the Mass: "I believe in one God…" The question of the Trinity is of the nature of God, not the number of gods. You could argue that it is false but you cannot argue that Christians believe in three gods.
Exactly, even those of us who aren't Catholic, Orthodox or Protestant still have this in common with all the others. We are monotheists just like the Jews. The Trinity deals with the nature of God, it explicitly states that God is One God and not many.
The Creed didn’t come about until way more than 200 years of fistfights. Tell me, how do you think non-Christians read this, the CENTRAL mystery of our Christian faith:
Contemporary Catholic: “I believe in God, the Father Almighty, Creator of heaven and earth; and in Jesus Christ, His only Son ...”
Latin (ca. A.D. 700): “Credo in Deum Patrem omnipotentem; Creatorem coeli et terrae. Et in Jesum Christum, Filium ejus unicum ...”
Anglican: “I believe in God the Father Almighty, Maker of heaven and earth: And in Jesus Christ his only Son our Lord ...”
The Trinity is not casually called the “central mystery of our faith” lightly. Oh, yeah, and our (small-”c”) christian friends, the Mormons, come right out and say there are three, not one.
Plus with the upcoming updated English/Latin changes to the Roman Catholic Church mass worship in the next few weeks, the wording for the Nicene Creed will be going from “We believe” to “I believe” in one God, which will work to help make Christian/Catholic faith stronger.