Should I take Calculus 3 in high school with no college credit, or should I take AP stats?
I would take Calculus 3. Stats is pretty worthless until you are considering stats within the context a specific field of study. You will probably need to take it in college, depending upon what you study. But the high school AP stats is pretty worthless and not really considered math (it is application of math) whereas Calc 3 is genuine math. Forget college credit. I’d urge you to take Calc 3 if you were in my family.
Calc III
If you take calc III in a college while at high school, your credit should be accepted by college. Though you have to double check with the specific school you are going at the end. There is a section in college application asking if you have any college credits. So if you could take it in college, it would make you look better, I think.
Calc III especially if you’re applying as a STEM major.