Interesting to know we only use 10% of our brain.
That's actually a myth.
So the idea of the Trinity, which is supported by the Bible, must be beyond human understanding
This is an argument from ignorance; you're supposing something you can't know as a reason to not know something else entirely. The logic does not follow.
if we could understand and explain everything God does and why he does it. It would make us smarter or better than God. What kind of God is less then the people he made?
Non-sequitur. Knowing how a god does something does not make that god less than, its a poorly pleaded qualification on your part.