Well, like most people (I hope), only in self defence, the same as if I were to hit a guy. I guess I might be a little less likely to hit a girl in jest, because they tend to be (this is out of the girls I know, don't shout at me for being sexist) a little more delicate. Besides, I don't have as many female friends that I'm close enough with to playfight with.
As for your situation? Whilst I don't necessarily think you getting involved in the first place was wrong, I think your retaliation was ridiculously over the top. He pushed you, so you hit him three times, leaving him gasping for air? How is that possibly sensible, and can you blame her for attacking you because of it? In that situation I don't really know if hitting her was right, you shouldn't have got yourself into that situation anyway.
As for your situation? Whilst I don't necessarily think you getting involved in the first place was wrong, I think your retaliation was ridiculously over the top. He pushed you, so you hit him three times, leaving him gasping for air? How is that possibly sensible, and can you blame her for attacking you because of it? In that situation I don't really know if hitting her was right, you shouldn't have got yourself into that situation anyway.