Marketo doesn't have a stand-alone form report, but form fill information (i.e. conversion numbers) can be found by creating a Landing Page Performance Report. In the report's Setup tab, specify the date of activity and then view the report to see the conversion metrics. If you export the report and then filter out the landing pages with "0" conversions, you'll have all of the landing pages, and by extension all forms, used in a specified time period. It will take some time, but you can go back and make note of all of the forms that are currently being used on those landing pages.
Also, you can vote on some ideas by clicking on links below:
Note: Approaching this way will only work for forms hosted on Marketo landing pages or forms that are iframed on your website.
I'd extract the Activity Logs via API for those 3 months and search that way. That's the way to get the results for all forms (embedded and named).