on my common app, i noted that i will be taking 3 ap tests, but i dropped one, and now i am only taking 2. should i let my college know that i had dropped one ap test? also, will dropping that 1 ap test cause anything negative, like rescinded admission?
You can inform them. But it is not necessary unless the college is a UC, which it obviously is not in your case.
Regardless, it will not impact your admissions decision.