In general, I would guess that you are going to want to add a project for each matter that you oversee. You can put an action in each project to make a note of progress. If you open the inspector window, you can set it to repeat, either "Repeat Every" if every 90 day period should have an update in it, or "Due Again" if there should never be 90 days between notes. It is probably a good idea to set a start date on that repeating action as well so it does not show up as being ready to be done again as soon as you check one off.
I personally think that OF is not the best choice for keeping track of what actions you have taken, as opposed to what actions you have to do. If you open up the view bar and set the availability filter to "Completed", you will see the actions you checked off, which may be a useful reminder, but I think it is easier to keep track of the work you have done in another application.