What should I do if my male beats my little female up?.?

I have three Roborovski dawrf hamsters. Two females (the first one is bought on Dec 11th 2010 she was 3 months old and the second one on Apr 16th 2011 and she was only 1 month and a half). When I bought the first one, Runnie, they told me that it s a boy (I asked for a boy) and, after few days I bought the second one, Raddel, I found out that they are both females. I wanted two have cubs, so two weeks later I bought a boy (this time for sure), Biggo, and he was 3 months old. They got along at first pretty well, normal fighting among them but after a week Runnie seems weaker than I knew her (she was pretty fat all the time even if she ran on her wheel and she always wanted to bite me or my friends) and I found a bitten mark on her belly and she was a little bit thinner but otherwise she runs in her cage and on her wheel. The little one seemed okey, I didn t notice anything strange at her, but I thought it wouldn t be a bad thing if I check her out anyhow and then I found out that Raddel had a lot more bitten marks on her belly and I got scared. I went to the vet with all of them...but they told me that they can t do anything, because they are too small and anyway they ll still fight and it s normal. I am afraid to let her around Biggo. I followed them up and I noticed that Biggo was the one which hurt her. He attacks her everytime he sniffs her and bites her. I don t know what to do for them to get along each other. Biggo and Runnie don t seem to have troubles and Runnie and Raddel niether. First days I bought Biggo they slept all together in their little house and I was happy for they, but then Biggo started to act strange and I don t want my little one to get hurt. I put her away. But I don t want her to be alone either. What can I do?.