You can get a better idea of this by looking at the requirements of the schools you're applying to. That said, I've never heard of getting two letters from the same guy. He can talk about both in one letter. As long as the 2nd letter is positive it seems fine to me. If the third chem prof will write a good letter, then it can't hurt to get it. I'm assuming gen chem was first year, so it might be good to get a letter from an upper year class.