triad, three at a time, threefold, a group of three
(from The Barnhart Concise Dictionary of Etymology)
So we agree that by definition trinity means three.
Do we also agree that there is one (True) God? That's good, because that is what the Bible says.
Can we further agree that God is one? That, too is good because that is also what the Bible says.
What? You say that God is three? Wait, you say that God is one? Or you say that God is both three and one?
To believe in the trinity means to believe that God is three.
To believe the Bible is to believe that God is one.
Sorry, I didn't write the book!