Okay, I’ll make it simple . . .
People like having more than one god, so Catholics believe in three. It’s fun, there is a kind of freedom in it. But it is heretical to be a polytheist, so they call this belief in three gods “monotheism” which is a belief in one God. So basically, the Catholic belief in the Trinity is a way for polytheists to call themselves monotheists, and still be able to call everyone else heretical.
Other religions that have exactly the same Trinity, are indeed called heretical by the Catholics.
It’s sort of the grown up version of “up your nose with a rubber hose”.
You don’t really believe that, do you?
Actually, you made it wrong.
People like having more than one god, so Catholics believe in three
Completely wrong; we believe in one God -- one divine substance -- who is manifest in three Persons (identities).
This is contrary to our ordinary experience, but not contradictory at all. (As Frank Sheed points out: you are a single person with a single substance -- your human nature. One person, one substance. A rock is a single substance but not a person. Zero persons, one substance. What is self-contradictory or illogical about understanding God to be three Persons and one substance? Nothing.)
The OT is perfectly clear that there is only one God. (Isaiah 43 and following. Try it on a Mormon sometime. They have no explanation for it.)
So Jesus did not believe in "three gods" when he said "baptize them in the name of the Father, and of the Son, and of the Holy Spirit". The only possible explanation for the obvious equality of "Father, Son, and Holy Spirit" in that expression is that they are three co-equal persons.
And that's the orthodox belief, "the Father is God, and the Son is God, and the Holy Spirit is God, but there are not three gods but One God. And the Father is not the Son, the Son is not the Spirit, and the Spirit is not the Father." (Athanasian creed)
Other religions that have exactly the same Trinity, are indeed called heretical by the Catholics.
If they have "exactly the same Trinity" they are Christians, and, while they may be heretical on other points, they are orthodox on that one.
As I said, you didn't "make it simple," just wrong.
sloppy theology
People like having more than one god? That is an odd way to talk about polytheists. Polytheists believes in many gods because they think there are many gods, many of whom they wish did NOT exist. I suggest you read Homer and Hesiod so you will know what you are talking about. If that is too deep, I suggest you rent the miniseries “Rome” where the special features give a good and easily understand discussion of Roman religion.