Set analysis to get number of buyers of the last 3 months

[cicciput]

I'm trying to understand how set analysis works.

It's not the syntax itself that puzzles me but rather how you can get results rolling on months.

For example I can get this to work:

=Sum({1

    <%SaleEndDate = {">=01/05/2014<=31/07/2014"}>

} Account.DCF)

which means count buyers whom purchased in May, June, July.

Now I cannot understand how to get a table where, for each month count buyers who purchased that month and the 2 months before, so:

Jan 2014 | Count buyers of Nov + Dec + Jan

Feb 2014 | Count buyers of Dec + Jan + Feb

Mar 2014 | Count buyers of Jan + Feb + Mar

...

Nice to have is having this related to months selected in the calendar, i.e. if I select one month I get 1 line, if I select 3 months I get 3 lines, if I select 2014 at the moment I get 10 lines and so on.

Thanks for your support

[Gysbert_Wassenaar]

Perhaps this document helps: Calculating rolling n-period totals, averages or other aggregations