How about
:
For the duration of the trip / meet, none of you three can pee without getting permission from one of the other two.
...and, that permission has to come with a 'price' of some kind attached. It could be as simple as 'kiss me on the cheek' or '5p please' - or it could be something waaay more amusing
Of course, if you don't like the price one person asks you can always beg permission from the third - but they pretty much have you over a barrel at that point...
Sound like the sort of idea you were thinking of?