[souffle]

You have been badly misinformed and I would go back to pets at home and tell them that you have been sold a large group which you should not have been and they are now fighting. I think you also need to get the sexes checked as the humping may be because you have been given a female as well as the males. 4 is far too many for any cage this store sells.
They seldom stay together past puberty and fights can be terrible. It;s not a natural situation for them and they can kill each other. The one being left aside now in the trio is likely stressed and could well be being mentally bullied and kept from food by the others. Any tiny change even like you handling one and not the others can set them off. With groups you need to be ready with single cages for them all in case. They are much better pets as singles and you can rest easy you are not going to wake up to carnage.
I'd never recommend groups as pets I'm afraid.
You can't neuter them as they are too tiny and I doubt it would help anyway. Small houses with no outlets can make matters worse actually as they can trap each other. You need to have really deep litter in the cage so they can dig down and you need to scatter feed so they all get some. Watch the sizes and if one is very small then he needs removed. Also try and check the sexes else you may soon have even more! Pet stores are notorious for misexing them. The store should provide you with more cages if you kick up a fuss but you need to decide if you want 4 cages in your home. Sorry not good news but personally I would separate them all now.