I've had two female mice for about a month, and i'm going to get another female mouse tomorrow, will my two original mice hurt the new mouse?

Mouse fighting? You mean as in Gladiator Mice? i LOVE IT!
I smell a reality show! =]

i don't think they will, ofcourse keep an eye on them to make sure they dont bully the new comer. But i dont think they will,female mice are very social with each other, i have 9 of them in 1 cage an they NEVER fight

They should be fine, but give the cage a good clean out (to remove scent) and rearrange to toys, house, etc. so that the cage is not so recognizable. Also, let them meet on neutral territory (somewhere that your current mice don't recognize as 'theirs'). Keep a close eye on them for a few days (including at night when they're most active) to make sure she's accepted.