If we decide to vacate this month .our lease ends 3/31/10. We were given our 3 day notice yesterday. Will we be liable, because they need a 30 day notice?

Well of course if you have not paid for this month, the pay or quit is for this mont, and you owe it,

Depending on the state ( if in US) you will either owe for the full month of April also for notice or at least though the day 30 days from the date you gave notice