It doesn’t hurt you to apply, unless there’s another (UC) school you would rather send your one waitlist app to. In the meantime, you should SIR to one of the schools you were accepted to
You have to balance how much you want to go there vs how much that will hold you back from fully committing emotionally to either UCSD or Davis. Those are both great schools and you don’t want to view them as the consolation prize. Ideally you could fully commit yourself to one of those 2 and then get a nice surprise from UCSB in a month or two. Maybe once you SIR, apply for housing, sign up for orientation, etc that you’ll invest yourself in UCSD or Davis.