My two girls have always shared, then when DS came along, we set the spare room up for him, he was barely in there before we had guests over so decided to chance it and put him in with the girls, he's still there 3 months later as they like having him in the room and do not want him to leave, they don't disturb each other apart from between 6 and 7 one will wake and the rest will magically wake as well especially as DD2 can reach the light switch

so I'm just leaving it until they are no longer happy with it
P.s. our DSs room has a single bed, with a pull out single bed underneath (it goes up to the same height) so when family come over from England they will have his room and he will go in with the girls (we will probably get a small sofa bed/futon type thing for their room so he has somewhere to sleep!)