That's the U.S. regs. Other countries don't get a third pilot until over 10 hrs. Don't know when they get the fourth pilot.
Most U.S. carriers have only one Captain and 2, or 3, FO's on board. All FO's are left seat qualified.
As far as who flies what leg? The Captain decides. The combinations are all over the map. Captain might not make any of the landings.