I don’t think colleges are expecting students to take all 3 science AP classes. As you point out, that is 6 science credits. I have not seen any college say they expect more than 4 years of science. My first kid loves science and managed to squeeze 7 science credits into her schedule but her sister isn’t particularly interested in science and will just do the basics plus one AP. Had not even occurred to me that any college would find that insufficient.