Level up more. Then learn to mix drinks. I hear the Quickstep, Painkiller, or Energizer are all very helpful in that fight. And then remember you only have to defeat one of them, so just keep going after one of them and it shouldn't be too bad.
Others have suggested getting the LMG or whatever the gun is that carries 300 rounds from the security office near the night club. Then just unload that on one of the girls and she'll never get close to you. Worth a try.
<sigh> I somehow made the mistake of saving right after preparing to fight them, leaving me with about 5 minutes before the case timer runs out, and it always runs out right in the middle of fighting them. Pretty much screwed, although I can **almost** beat Amber before time runs out, but they keep kicking my ass.
Wine + Orange Juice makes QuickStep. You can make it in the Atlantica Casino, just bring OJ since they have Wine already. Out on the Silver Strip, is there is maintenence room next to the Pawn shop, go there and you can make the bowie knife + boxing gloves. This combo works well