The only reasoning I can come up with for why all three kids had to be Gryffindor is because it would have been a lot harder to pull off the stuff they did if they had to sneak into and out of each other's houses all the time, or if they didn't automatically have nearly all of their classes together. But you're right, according to the requirements for each house, Hermione could easily have been Ravenclaw, and I could see Ron as a Hufflepuff.

Actually, I remember one book (the one with Umbridge, I think?) where a Ravenclaw asked Hermione why she wasn't in their house, and she said the Sorting Hat seriously considered putting her there. I guess her courage won out?