Choosing reference letters
Having sat (as a graduate student) on my department's admissions committee for two years, I can provide a few thoughts from my own personal experience.

First off, everyone has "good" references - it's very rare that a reference writer will say something negative or even something neutral. What sets apart the really good ones is that the writer can provide a specific, concrete anecdote that demonstrates an applicants skill set, attitude, previous successes, etc. So in that regard, knowing you the best probably trumps almost everything.

That said, having three letters from postdocs (none from professors), all of whom are in the same lab, is likely to make your letters somewhat redundant. If you worked with each of them on very different projects, or you know that they have very different things to say about you (in particular if they can speak to different aspects of your work or personality), this might make sense - but to have all three from the same place might be a bit much.

Not knowing anything else about your situation, it would seem that perhaps two letters from your current lab and one from a professor at your undergrad institution would be a reasonable balance.