1/3, NO MORE NO LESS FOR EACH logically speaking... if there are three persons who are living in an apartment with two bedrooms ..obviously two are gonna occupy one room and the other room being occupied by the third..now does that mean that the former two is gonna pay just 50% of rent in total and then the latter fellow the remaining 50%..no way ..if it were to be work that way ..than all of them would want to sleep in that room that will accomodate two people inorder to pay HALF the rent as compared to the third.. now in this case, the only difference is that the couple would obviously share the room leaving absolutely no option for the poor fellow. he does not have that choice to occupy the room he wants, so why should he pay 1/2 instead of a 1/3. he would gladly share the room with either of the two( which is not possible in this case) if given that option. many are raising the point that if he agreed to pay 50% in the begginning he should stick to that..but again ethically speaking, at first, he might in an error of judgement or by mere foolishness agreed to pay 50% but might have realised later(that it was'nt fair enuff) by rethought or someone telling him. and is the third fella the regular cook aswell, if so than he should charge the couple extra, unless ofcourse the couple is regular dishwashers ..lol