[PeteMcCracken]

Well, assuming both pans are the same depth, there will be three, 3"x5" pans in one 9"x5" pan.

1 cup = 14.44 cubic inches, so, roughly, a 3"x5" pan will hold 1 cup per inch of depth, approximately while a 9"x5" pan will hold, approximately, 3 cups per inch of depth.