Last 3 month month year

Hi Experts,

       My dateformat of my field is MM.YYYY,

so in the prompt for end date i used formula

          userresponse("Enter last date:") as in prompt statement

Now i want the last 3 months date

e.g; last month =12.2014

then i want 11.2014,10.2014,09.2014

Thanks.

Ashwin