Navy Adversary
In this situation I assume that the Pilot resting would have to be woken up and return to the flightdeck if one of the other 2 members HAD to have a toilet break.
FYI under many of the rule sets regarding relief/augmented crew Operations the "off task" pilot(s) has to have the opportunity for (to paraphrase) "a continuous period of undisturbed rest", so as has been pointed out, no, you don't get the off watch pilot back on the Flight Deck to cover for toilet breaks - those have to managed using the same company procedures as would be used on a two pilot sector.
As far the comments about who is as PIC goes etc, FWIW where we are there is one aircraft Commander designated for the entire flight, and he/she is the PIC is Commander regardless of where he/she is seated during the flight. Who is "acting PIC"/P1/P2 on the flight deck is another matter.