Heh - there's definite history there; strange, complex three-way history, I wouldn't mind betting...

I always think of Doctor/Rani as being some sort of student fling; the clever, focused girl and the boy she thought was kind of loveable in a quirky, disorganised sort of way. Eventually, his lack of motivation annoyed here, and she decided she could probably do better. The Master was sort of the third wheel, unable to decide which of the two of them he had the biggest crush on (Doctor, he eventually realised, I think...)

Hmm, that was almost a fanfic in itself...