some schools require 3 science professors, but I can't find the 3rd one, who can write me a good reference letter. Can the post doc who is working with me in the lab write one for me?
Usually the LORs need to come from someone that has been a prof in a class you took. Additional LORs can be a positive thing but you need to make sure you meet the LOR requirement for your schools or the additional LOR does you know good. Some schools need a committee, some need three LORs, some need two.
i had a post-doc write a letter for me, but she was the instructor of my biology course. i don't think getting a post-doc that works in the lab with you will work. you can, however, get the PI of the lab to write you a letter 👍