Yossarian wrote:
Hypothetical situation: I rent a plane and fly 2 friends to city A. I drop
them off and fly back. If I pay 1/3 of the rental time and costs going
there and full amount of the return leg, that's legal, right?

Depends. Why did you fly to City A? If your sole motivation was to
convey your friends there, the legality is questionable. It could
be perceived as an air taxi service.

If you had a personal reason to visit City A (eat the World Famous
Braunschweiger, visit the museum, visit friends) and you're simply
taking your friends along but needing to return home before they
do, that's legal with expense sharing as you describe.

Cheers,
Sydney