Choosing reference letters I'm applying from Europe to US or US-styled Ph.D. programs and I'm wondering about who to choose for referees.

I graduated 2 years ago and my undergraduate professors don't remember me well, or at least I don't think so. In those two years I have worked as a research assistant in one lab, with three different post-docs. Should I diversify the source of my references and try to get references from my undergraduate institution or should I just opt for the people that I've been working with?

I think the latter group knows me better and has, generally, a better opinion of me and my work. Then again, I'm afraid that the single source of references will be suspicious.

I mentioned having studied in Europe because the undergraduate research options are typically limited, compared to American institutions where research experience is strongly encouraged. I only did a research thesis during my undergrad.
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.