This can get problematic. Starting point: if he's an aircraft owner and all three of these guys are, say, business associates, they can basically split the gas, oil, and parking three ways (direct operating costs). From this point, conditions and interpretations become divergent. Here we have an operator (who I assume doesn't have an air taxi license) essentially renting an aircraft and pilot to a couple of businessmen. Yeah, I know it seems to be set up differently, but I think this is how the FAA would look at it. In this case the definition of "sharing expenses" is open to interpretation, and the FAA's may not be the same as yours. BE CAREFUL. Over the years I've heard of a few violations in cases like these. Some were obvious dodges; others were legitimate attempts by folks to comply with the rules as they saw them. The fuzz saw differently. You may find this isn't worth the trouble.

An aviation attorney with experience in these matters can give a definitive answer; I ain't one of those, so this is the best I can do. If you're set on doing this talk to an expert face-to-face; this forum isn't the place for legal advice.