Joe post a package (twbx) workbook with a bit of sample data and one of the text table guys are more likely to take a look.
If I understand correctly, two calculated fields can be created. One for this month's sales:
if [Order Date]=dateadd('month', 0,today()) then [Sales] end
And one similar to the following for the last 3 month's sales:
if [Order Date]>=dateadd('month', -3,today()) and [Order Date]<=today() then [Sales] end
Hope this helps!