See that table of hot girls over there by the jukebox? They're my best buds. Don't worry, they know who you are. I've told them everything about how our last three dates went. As a head's up, I've also divulged both your quirks and your favorable attributes so they have a few pre-conceived notions about you already.
Now that you're standing in front of them, they're judging every single thing about you with alarming speed: your hairstyle, clothes, shoes, and general affability are being computed at a rate that would make Deep Blue's head spin. I'm sure you know all this, I'm just reminding you.
My question: do you have a plan? Don't buy them all a round of drinks right off the bat. That'll look like you're kissing up. Don't talk too much, they'll think it's weird that you're taking over the conversation. Conversely, if you're too aloof, they'll think you're an asshole.
My advice: smile, be pleasant, and ask each girl a question about herself. Be sure to make eye contact when you talk with them! And, don't talk about either sports or work unless someone else brings it up first then keep your answer short.
And, it'd be sweet if when you finish your beer you announce that you're going up to the bar and ask if anyone would like a drink while you're up. Then and only then can you buy my friends a drink. Don't let them pay; wave it off as you hand them their drink. Also, if you buy me my drinks and keep me happy, they'll notice and appreciate it. If you do that, you should be fine. Trust me, they'll love you. How could they not?