Hi L.,
We've done quite a bit of sharing, so even though I don't know what to tell you in your exact situation, I can tell you what our experience is. Right now, thankfully, all three of our kids are excellent sleepers. We worked hard on that, and I am glad it paid off. (As a side note: have you read Healthy Sleep Habits, Happy Child? It has ideas for sleep training even for older children, so you might find some ideas to help correct your oldest's bad habits.) Our kids are a girl (7), a boy (2) and another boy (1). When the youngest was born, he started out in our room. Then he got the nursery to himself while the older two shared a bedroom, so that his waking at night would not wake them. This turned out to be a problem as they tended to keep each other up late talking and making noises rather than going right to sleep. So eventually we moved my oldest into the nursery with the baby. I was worried about my older boy climbing into the crib, so that is why we chose to move her. That worked pretty well (I think the baby must have been sleeping through the night at that point, so I didn't have to worry about him waking her, though even if he did wake up, she is a pretty sound sleeper, so I don't remember it being a problem the majority of the time.) Most of the time she was quiet enough when she went to bed, not to wake him, but there were occasions when she woke him up and we had a crying baby for a bit.
After awhile, we put her back into the bedroom with my 2-year-old, because the one problem I had with him sleeping alone is that he would wake much earlier than the other two. After I got tired of trying to head him off before he went in and woke them up, I just moved her back in with him so that he could just wake her up and not the youngest (now about 1). My daughter was not thrilled with this arrangement, but it has worked out OK.
Just this last week, we decided to use the nursery as a home office for my husband, so we have actually now put all three kids to sleep in the same room. We moved all their toys out into the family room where they always end up playing anyway. This means less mess to clean up, and has really worked wonderfully so far. Their bedtimes are all 1/2 hour apart, so one is asleep before the next comes in (in theory). Some nights they wake each other up and are up later than normal, some nights it works smoothly and they are all asleep when they should be. They all wake up at a decent time and can play quietly in the room (without toys amazingly) until it is time to get up. I still have to go in and get my daughter up each night to take her to the bathroom, but so far that hasn't woken the boys up at all, since it would be the normal time for me to check on them anyway.
I don't know if any of that helps. I know it is a slightly different situation since they are all good sleepers, but as you can see, putting them together has not adversely affected how well they sleep in the long run. Feel free to play around with it too. If one arrangement doesn't work out, then switch it around.