[nikkianni]

I don't see the need to do any disciplining over that. I can't think of a 3 year old I know or have ever known that hasn't come out with that one from time to time. And often the pretending to cry/letting them know that wasn't nice thing works better than anything else. Kids that age generally respond to that better.
As far as who should do the disciplining: I think I'm in the minority here, but my family was very close when I was growing up and we remain close now. We're very much It Takes a Village I guess you could say. Though I don't think anyone's actually heard of that in my family.
We all do the disciplining. My kid, BIL's kid, a cousin's kid, doesn't matter. Whoever's nearest if a parent isn't around disciplines. Of course that also includes reporting the bad behavior to the appropriate parent.