Please try the steps below:
1. Create a calculated field: the last day of the month:
2. Create a bin for the sales on the last day of the month
IF [Date] = [the last day of the month]
THEN [Sales] ELSE 0
3. Drag [Date] to columns, select month (e.g. May 2015); and drag [Sales] to rows
4. Create table calculation for Sum[Sales] --> Moving average --> Previous 3 months
Hope this helps.