I apologise, I hadn't noticed that the question is tagged Excel 2007, the answer I initially gave will not work with older versions of Excel. The following updated answer should work.
You can easily do this by adding a helper column (which you can hide if you don't want it visible).
Assuming Participant ID is in Column A and Date of Event is in Column B, and that the headers are in Row 1 with the data starting in Row 2, insert a new Column C (which you can hide if you want).
In cell C2 enter the following formula and copy it down all the rows of data.
This will return the participant ID followed by the month and year of the event.
Now enter the following formula in cell D2 and copy it down all the rows of data.
=IF(COUNTIF($C:$C,$C2)>=3,"Participant "&$A2&" has three or more events in "&TEXT($B2,"mmm-yy"),"")
This counts the number of rows where the Participant ID and month are the same as in this row (in other words the number of events this participant has in this month); that number is three or more, a note is displayed.