How about something like this?
3 month: if [Actual Date] >= #2015-12-01# then [Amount] end
12 month: if [Actual Date] >= #2015-03-01# then [Amount] end
36 month: if [Actual Date] >= #2015-01-01# then [Amount] end
You can of course change the dates depending on your need. Then instead of time group on columns, place measure names on columns, measure values on rows and turn on ref lines per pane:
There may be a better way, this is just what came to my mind!
Thanks for the quick response.
While this wasn't the outcome I was looking for, your answer led me in a direction that got me closer.
I still would like the Average lines to include all dates in each Time Group and show across all dates in each Time Group.