Do you mean that you have many records and on each detail record you want to show the current avg sales for the past three records? This can be tricky b/c you have to keep a rotating list in memory of the what the last three were so that you can subtract the fourth one in the past. But if you just want to keep a running total and not include a value if it is zero, then I would do a manual running total that tracks the total sales so far as well as how many records have been counted. If a record is zero, then don't increase the counter for that record.

So which method are you trying to do for this report? The first or second one.