Please note that your name does not appear on the lease. The house / apartment belongs to the three girls. It could be that you're over there all the time, so the other two girls feel they don't have any privacy or feel they could invite their friends over, because you are always there.
It could be you act like a jacka$$ and make them uncomfortable. It could be that you eat up everything in the house and don't replace what you've eaten.
It could be any number of reasons. If you're curious, why don't you ask them if there's something that you do or say that bothers them? One day, you could be in this same predicament so this will be good training on how to get along with everyone.
Make an effort - find out what's going on. Don't put your girlfriend in the middle of this. After all, her relationship with you could end tomorrow; she will still be stuck there until the lease runs out.