3 day notice to quit?

Wow...this girl is going to make out like a bandit - 3 months of housing for the price of 1 month's rent...not a bad deal. I suppose she's not returning phone calls either, huh? Since she never fulfilled her obligation to provide a security deposit OR paid the 2nd month's rent she essentially gave her notice to vacate (at least that's the way I'd see it!) If it were me I would just show up at the house and hand her a notice to vacate telling her she's got til the end of THIS month to get out - how's she going to fight it?