It is possible to setup a schedule for every 3 months and you were on the right track too!
Here are the steps (in reference to the Time tab of the activity setup):
1) Times: Set the radio to the 'Custom Schedule' option and add the time (or times if you want to to run more than once on the same day) to the adjacent listbox using the ADD button.
2) Days Of The Week: ensure all checkboxes are 'ticked' for each day of the week.
3) Dates: add into BOTH the 'Start date' and 'End date' columns the (same) date you want your activity to run. For example: if the activity needs to run on 1st June 2010, add an 'Include' row setting the Start date AND End date columns to 1/6/2010 (or 6/1/2010 if using US date format).
Hope this helps!
Thank you very much for your answer.
I didn’t see in the Time tab (in Dates), it was possible to add many 'Include' rows with the same "Start date" AND "End date"columns.
My problem is resolved. Now I can setup a schedule for every 3 months :)