Creating Rolling Sums in Reports

Question

If the underlying database does not having rolling sums computed for you, how can you report rolling sums?  I tried to use the runningsum function but have no dimension to make the sum reset.  If I have a table in the report like:
Customer   Month   Sales 
A               Jan       1
A               Feb       1
A               Mar       1
A               Apr        1
A              May        1
A              Jun         1
B               Jan        1
B               Feb       1
B               Mar       1
B               Apr        1
B              May        1
B              Jun         1
How do i get a running 3 month sum by Customer like:
Customer   Month   Sales   3 Month RunningSum
A               Jan       1           1
A               Feb       1           2
A               Mar       1           3
A               Apr        1           1
A              May        1          2
A              Jun         1          3
B               Jan        1          1
B               Feb       1          2
B               Mar       1          3
B               Apr        1          1
B              May        1         2
B              Jun         1         3
Thanks,
Jimmy

Answer 1

I found a solution. Create several variables such as Previous1Month = previous([MetricName])
Previous2Month = previous([Previous1Month])
Previous3Month = previous([Previous2Month])
PreviousXMonth = previous([Previous(X-1)Month])
To create a rolling 3 month, you just create another variable
Rolling3Month = MetricName + Previous1Month + Previous2Month
Of course this becomes very tidious with lots of metrics and variation of grains(wkly, monthly, quarterly...).  That's why you would want your underlying data model to do this and have the etl do all the dirty work. =)
Thanks,
Jimmy