R.L.
If it were me, I would have the 2 older children share a room and the baby on his own. You can always transition them later and allow the oldest to have his own room. I agree with you due to the crying issues and midnight feedings I think it would be easier to have the baby alone so that you don't wake up the toddler and have a cranky toddler all day. My son was in a twin bed at about 2 1/2 so I think going with a twin bed would be fine. You can always get a rail for the twin bed if you are afraid he will fall out. Do whatever you think will work best, as they grow you can always change things around.