I had a similar senario. Bonded 2 and 3 year old boys and introduced a 6 year old boy.
It went really well and fast became friends and huddled together all three in a sleeping pile. After a few months the new one started attacking the old ones to such a degree I had to take him out. Now he can't even have play time without fighting
It's reached a point where I consider rehoming him cause he seemes distressed and anctious, wanting to get in to their cage to fight.
It might go well with your girls, but it might be safest to only give them supervised playtime, as when it goes bad it goes BAD! I almost lost one of my boys to the fighting, they can kill eachother even months after bonding.
Mine is just one experience though, I'm sure there are others who have made it without a problem.