The dark magus sisters was the hardest battle in FFX(after Penance).
You can confront them one by one if you want to fight them,otherwise summon Yojimbo with grand summon to use zanmato.I'm not sure you'll be able to use it though.If they catch you all three of them they'll have the first move and they will use their overdrive,so then it's bye-bye.
So the only option is to confront them one by one(or two,then one).You must have high stats,and definately ribbon,auto-haste,auto-phoenix.