You wont need much of a opener. You know 3 girls out of 6, right? So you already half of them, or let´s see it the other way around, They know YOU!. Don´t act all weird, they will probably see right through you.
It´s a piece of cake "opening" on party´s (way easier than in a bar). You always know someone at the party, otherwise you wouldn't be there. When someone you know talks to someone you don´t know. Simply go in to the conversation, say hello to your friend and use the line "introduce your friend, it´s the polite thing to do" (this is from style or mystery, i have been reading to much, can´t really remember where i read everything). I always get the "oh sorry, of course, This is ......" Hand shake and a look straight in to her eyes and don´t break that eye contact, it should feel a little bit weird, but she´s the one that has to look away:)
2. Honestly, if you know you're gonna be drunk you will or sure go to bed don´t even thinking about the girls. If you do well at the party, you will be going to bed with one of the girls as well

If you´re not that drunk, all i can say is Confidence! it´s all about confidence!