We have just done this exact move not long along. We decided to put DD (nearly 5) and DS1 (2 1/2) in a room together and DS2 will be in a room by himself when DH finally convinces me to move him out of our room, LOL!!
We decided with this combo for a few reasons:
- so the baby doesn't disturb the older sibling overnight
- we read books in bed of a night so didn't want to potential wake the baby
- we wanted the older sibling to be able to play in their room during the day
- we brought a set of bunks so the older siblings ages work better with the bunks
- plus they wanted to share a room
When DS2 is ready for a bed we will put the boys in together and DD will have a room to herself. That is about 1 1/2 - 2 years away.
Good luck with the decision.