As far as being in a relationship and cheating, what your partner doesn't know won't hurt them...
J/K. I know that when you drink you tend to get more, "outgoing" and you'll be more likely to talk to people. You can do/say things that you wouldn't say sober. I know, because I've been there.
I say since you were drunk, you now strongly regret it, you should just forget about it. What you did was nothing huge by any means. It's not like you made out with those girls, they were just little kisses. You can kiss your mother that way.
Now if you made out with a girl or had sex, then you'd have something worth REALLY feeling guilty for. I'd say just leave it be, and don't do it again in the future. Be careful. When you're drunk, you still have a general idea of what you're doing. Just think about it next time. Telling her will probably only make matters worse.
Depending on what kind of person she is. No one wants to hear about their BF/GF kissing another person. Regardless of the kind of kiss that was done. Which in your case was a peck on these girls.
I know as much as you'd like to think that she'll be happy with your honesty, and respond with open arms for telling her, it just doesn't work that way. She has feelings for you, and by telling her it can make it seem that it doesn't mean as much to you. That meaningness being her.
Move on and learn.
