Firstly - yes, this sort of thing will be easier in PT4. We have removed the distinction between session-based stats and reports (e.g. the Winnings tab) and hand-based reports.
To group other "stat" reports by month you'll need a custom stat - fortunately I have already built it, and I've attached it here.
If you extract the .pt3stat file from the zip and import it to PT3 (from the menu choose Configure > Configure Stats and on the Statistics tab click Import) then you will be able to use it in reports showing player stats, or custom reports in the Holdem Cash Player Statistics section.
Bear in mind if you add it to the built-in reports you will get a row for each month for each line which is already in the report, so a custom report is probably going to work best for you.
Tutorial: Custom Reports and Statistics