J.A.
Hi B.,
I guess you have to decide if the third roommate is going to lead to overcrowded conditions.
If your daughter is getting the larger of two bedrooms and a larger/private bath then she probably should pay a little more than her other two roommates since they will likely share a bath and one of the girls won't even have her own sleeping space.
It wouldn't seem fair that your daughter would only pay 33% of the rent if she is getting more of the space. The bottom line I guess is do you really want the additional roommate at all, afterall this was kind of sprung on you and does change the accomadations considerably.
Best Wishes,
J. L.