We have a 20 month old (already in a toddler bed) and I am due in 6 days. They will be sharing a room for at least a year since we are renting until we know if we want to stay in this area or not. (They are both girls.) The new baby will be staying in our room for a couple of months though in a bassinet though. At such a young age, I don't think it is such a big deal, but I wouldn't make your 8 year old share a room with anyone.
I agree that it would probably be best for the two youngest to share a room and the oldest to have his own. However, you might want the two oldest to share a room until baby is sleeping through the night or put baby in your room until then so you're not constantly disturbing your other child, unless of course he's a heavy sleeper and you know it won't bother him.
I have a 4, and 2 year old. And only 2 bedrooms.my 2 year old is such a light sleeper and we've had his crib in our room since he was born because he was in the nicu for the first 3 months of his life. Since he's gotten older my husband and I sleep in other rooms, I sleep in my 4 year olds room and hubby sleeps on the couch.
If I was deciding who shares and who gets their own room, I would let the 8 year old have his/her own room and have the 2 younger ones share. The age gap makes a difference...