they are very curious creatures, so you cannot leave them alone in the same room with the baby. I'm sure you're already pretty well baby proofed, so hopefully you'll have a few safe places you can leave your twins while you go up/down stairs to do something with the baby in tow. Have lots of places around the house where you can set the baby down, like in the bathroom, laundry room, etc. You should have lots of extra bouncies, since you won't really be able to keep the baby within arms reach of where the toddlers are. The good news is, the babies learn to sleep through noise really well!
My DD is fantastic and really could not care less. Now that Julia is more interactive they like to play together. C was all about helping and kissing and holding Julia from the first second.
Re: How's your 21 month old w/ your nb?