i was talking to a set of 3 girls and a fourth one comes in and says
shell give me her number if i get her a drink
i told her why dont you get me a drink and ill decide whether or not i want it.
Question:
1. Why did she ask me that?
2. Was she just saying that to give me her number?
3. What should i have said in this situation?

There is more than one possibility. Either she likes you and has old fashioned values of the man buying a drink or aka being a gentleman. OR she is gaming you to get a free drink. I have a few girlfriends who do this to guys when they're out and they do it just for free drinks, I know this because they've told me and plus they don't have jobs.

Kind of depends on the specifics...Was she over-hearing you asking for the other girls' number and just thinking you're desperate to get a phone number and saw an opportunity or did she come across the room and say that because she was genuinely interested in you and just wanted to sit down with you? Sounds like she might be a gamer...but knowing how things looked to someone standing at a distance, her body language and tonality in the situation is the key to knowing for sure. Still I think the neg was too hard...maybe I would have come back with something like "what is this? ...'let's make a deal' Just give your number."

Getting a number doesn't mean getting a number that will be worth anything.
The nickname for this is "wood", as in waste of paper - phone numbers that won't lead anywhere. She won't pick up, etc.
She just wanted a free drink. Shit-test.
A good comeback would be:
"Do guys SERIOUSLY buy a drink for you to get your number?" then look away with a grin as if you were amused by the idea and you found it ridiculous at the same time. It will make her feel uncomfortable for using guys for this and it will DHV you for not playing her game.

I would of laughed and said any/all of the following:
-i'm not that desperate
-if you wanted to talk to me you didn't need a silly excuse
-are you flirting with me?
-yeah sure wait right here ::leave::
-you got it all wrong YOU buy ME a drink, thats how it works.. honest!
-sure? ::ignore::
-sorry i have a girlfriend

well you get the idea, its one of my sticking points to never comply to a female especially if she is a stranger and even more so if she wants something.

Some of these don't give enough DHV, or right out DLV you by not being funny or making sense. So how desperate are you? Not bad, but doesn't quite feel right. Her reply will be "Umm, no?" If they were bitchy, that's not bad, but otherwise, it's just dropping a set. It's giving up, while also being rude with them. This one's not bad, but not that funny imho. What is this trying to achieve? I don't see the point in this either.

Do you mind elaborating what you are trying to achieve with these? I don't quite get it.

"Now if only I wanted your number." Chances are she's trying to scheme a free drink whilst making you look a fool (albeit maybe innocently). Whenever there are other girls in the set, you definitely want to deliver it with a smile, or you'll seem insecure or like an ass.

No doubt Beetle covers it here...

I vote with Beetle too. Im not sure if that number would have lead anywhere. It really would have been one extreme or the other. She either wanted a free drink (most likely) or her friend was trying to help you/her out by trying to force you to give it to her.

The quoted above is how I would handle it too. It's totally a shit-test and a scheme to get a drink. To be honest, O.P., your response wasn't half bad. You did good.

There is always the chance that she was interested in you and actually wanted to give you her number. Even so, buying her a drink is the chump way to get it and, looking at the big picture, it will lower your value. If she really is interested and wants to take things that direction, the drink is not the deciding factor here. And if for some reason it is, then you don't want anything to do with her (or I wouldn't).

As far as my responses, like I said I do not buy women drinks that I do not know already. It's just a rule i go buy why am i going to be that chump she gets a free drink out of? If she was genuinely interested in my why would she need a drink she can flirt and we can begin the game. The situation just screams "I want a free drink" and not an actual set i would want to open. So that is why some of my responses are sarcastic/blase.

When I engage a set I make sure it is on my terms not theres, why would i comply to their demands? I know some people don't mind buying drinks for a girl at a bar who they are trying to game, even then they get to pick their targets. Their targets don't demand a drink before gaining anything if they do most times the set is dead in the water so why would it matter?