dogbone

Hey guys I have read a lot of good tips on here about applying and I appreciate your guys' help. I had a quick question, how important are the reference letters? I have one professor who I do research with and will be able to write me one rockin letter but I have no other professors who are familiar with me and my work (I should have been better at that). So my second letter would probably be my supervisor from an internship and for my third letter, I am not sure. I am just wondering how much emphases the reviewers put on the letters? Thanks