interviewing for postdocs with two groups collaborating in the same grant, should I mention it? I have a skype interview for a post-doc at a big research group with PI-A, who is the leader in his field and would be a great opportunity if I got the position.  

At the same time, I have been in conversations with PI-B and PI-C about a co-mentored post-doc to work on a recently awarded grant. The work for the grant will be done in collaboration with PI-A. The grant describes a post-doctoral position for the laboratory of PI-A to do the work for the collaborative project under PI-A,B, and C. 

My questions are:

Should I mention my conversations about the project in my interview with PI-A's group?

Should I mention to PI-B and C that I am interviewing for PI-A?
I would start out by assuming that all of the PIs will know if you apply to any of them. If the conditions for the various positions (pay, etc) are similar then I see no reason not to mention it. Perhaps the three of them together can find you a slot, even if one can't. This would probably be different if the pay and such were quite different. 

And note that working with a top researcher is only sometimes better than working with others. The top person may have less hands-on participation at that point in their career. 

On the other hand, there can be issues with non-tenured supervisors, who may have little time for anything but getting their own tenure. 

But the collaborative possibilities you mention sounds like a big plus.