Single application for several postdoctoral positions
I don't think you can assume that one application is okay.  I can certainly imagine situations where the lab would want a separate application for each position.  Maybe they are going to be reviewed by different people or committees.  Maybe they have complicated HR procedures that you don't know about.  Maybe when reviewing applications for position X, they want to see an application that specifically addresses your qualifications for that particular position, and doesn't mention other things that are only relevant to position Y.

So the safest course of action would simply be to ask.


  Dear Professor MMM,
  
  My name is NNN, and I am a graduate student at UUU, working with professor SSS.  I see that your lab has some postdoc positions open; I believe that I could be a good fit for any of the positions XXX, YYY, ZZZ.  (Briefly explain why.)  Would you prefer that I send a separate application for each position, or a single application addressing my qualifications for all three?


This could also get you other useful information, such as "I think you would be a better candidate for XXX; for the YYY postdoc we are specifically looking for someone with expertise in BBB."