How to get three recommendation letters while I only worked with my supervisor? I am a Ph.D. student who will get a Ph.D. degree in less than 8 months in biophysics. While I am looking for postdoc positions many of them require three recommendation letters. One of these letters will be provided by my supervisor, however, what can I do about the other two?

I collaborated with other researchers on my projects, however, the collaboration was kind of at a distance while the collaborators were in another country. 

Is it possible to ask these collaborators for a recommendation letter? How could they evaluate me while they never worked with me directly or in person? They usually provided samples or commented on the draft I wrote, however, I did not interact with them the way I discussed the project with my supervisor.
These collaborators are exactly the persons who can evaluate your work, your person and so on really good and in the eyes of a hiring institute, are more objective, because they haven't been your supervisors. So, definitely write and ask them. Probably, they don't know about your future career and current projects, but, I guess, they want to talk to you again about this (via phone or skype) and then you can explain much better what do you want, what's your motivation is and so on.  I would also guess that one of them needs some help in formulating the LoR and then you assist with some bullet points (there are quite a few questions and answer here, for instance this  or that question with answers/comments).  So, go for it and good luck with it!