Hi, iam also going thur the same thing here, i have karleigh, who is almost 3 & charlie who is alomst 3 and joey who just turned 2, now i have molly 12 wks. they all still adjusting, when the older ones play they can get a bit rough, there has been a few growls , but i let them no right from the start that she is here to stay and be nice.I supervise them at all times, i make sure they all go out together and eat at the same time, get treats at the same time....the hard part is having 4 on my lap at one time
not a easy thing to do, but i do it,i show them all the same attention
good luck to you.