Rule #1 - Laws vary from state to state, county to county, even town to town. What we tell you here may not be valid in your area. Check with local law enforcement or self-defense expert before proceeding.
You can ONLY draw when your life or the life of someone around you is in imminent peril. That means a reasonable person in your position would think that there was going to be harm done. If you allow your weapon to be seen at any other time, it's brandishing.
You don't have to immediately fire once you've drawn (though many gun rights advocates will tell you it's the safest idea when faced with later litigation because it's easier to prove you were in fear for your life). If you pull it out and they all run, you're fine. However, once it's in play you must shoot to kill...anything else and the court will argue you weren't afraid enough to need to use it.
It sounds like you may not have extensive training. Get it. Now. Seriously, contact the NRA, or other private agency and get some classes in. I'd suggest Refuse to be a Victim, as well as a basic handgun course, and probably a weapon retention class as well. I'd further recommend verbal judo (or other vocal de-escalation technique). Most localities have someone that will provide a concealed weapon class as well, to familiarize you with the ins and outs and answer any questions you have.
Now, about your suggested scenerio. Let me tell you how I personally would handle it, and keep it legal. We're walking on the street and people come up to us. I position myself slightly in front and to the side of her, keeping as many as possible in my field of vision...preferably with hard cover on one or more sides. We stop walking 25' or more from them to judge their actions. If at this point they infer that they are going to rob us, I begin backup up while telling my gf to call 911. I turn 3/4 from them and (keeping my hadns in front of me in plain view) tell them that we don't want any trouble, but we will not be giving them anything, that we're calling the police and if they attempt to harm us I will defend myself. If they have no weapons, and they push towards us, I really have very little choice by melee combat, though I will strike to incapacitate (eyes, throat, temple). If any of them have anything I can consider a weapon (ANYTHING), I draw, aim at center mass and shout for them to throw down their weapons or I'll fire. If they don't, I kill every single one of them. If it goes physical at any point, I tell my gf to run like hell to a public place and wait for the police.
The point of this is that I don't draw unless I can prove iminent threat. That doesn't mean I have to be a victim and give them my money or car. I can refuse. Then when I refuse, if they try to make it physical I have the right to kill them. See how that places the responsibility on them? VERY important to keep this in mind.
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