My boys shared a room until they were about 7 and 9. Sometimes they fought...and one needs more sleep than the other and his brother was always waking him up to play when he needed more sleep, which made him insane...
But I think the hardest thing was just having their stuff and space joined, so that whatever happened with one, affected the other. We couldn't send one to his room as a consequence, without it also becoming a punishment for his brother (either trapped in there with an angry sibling, or unable to be in his own room at all)...also if we tried to withhold remove something from one, same thing, the other got cheated, because they were used to sharing everything. Send one to bed early, same thing!
So when we were finally able to separate them, it helped a lot.
My husband says having three kids is exponentially harder than having two kids. Instead of 1:1 play it becomes 2 against 1 play, and the adult/child ratio is no longer even, so there is always one kid loose while you try to corral the other two!. But if you really want a third, and think you can work around the room issue, go for it!