Best advice is get them used to going down at the same time and hearing each others cries. It will take some time for all of them to adjust but trust me it will be worth it. I have 2 that nap together since they were 9 and 11 months old now when one cries it doesn't bother or even wake the other! They are 20 months and 18 months now!

I would also suggest napping them once a day for a 2 hours at least!

Good luck!

P.S since its on the same level as the play room have you thought about using a fan for white noise so it will block out other noises like a vacuum etc...