Hi and welcome to Binky Bunny! It would definitely be best to keep the bunnies apart at all times. Even letting two or all be together for a minute could result in a fight with severe injuries. Rabbits can also hold grudges if they fight and hurt each other, which can make future bonding very difficult. It doesn’t matter if they were litter mates and have been together since birth. Once puberty hits and you are dealing with hormones, everything changes. And, you need to be absolutely sure that they are all females. Most pet store personnel know very little about rabbits and have often told people they were getting the same gender, then they were surprised with a litter, a few weeks or months after bringing them home.
There is a chapter of the House Rabbit Society in NYC. They have a vet list on their site, but you could also contact them to see if they know of any low cost clinics. http://www.rabbitcare.org/contact.htm
Here are 2 good articles on bonding:
http://rabbit.org/faq-bonding-multiple-rabbits/
https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/9/PID/940/Default.aspx