I think the easiest way to get that info would be to write a report in report writer. I think one of the canned reports under Historical CPU and Memory Reports is called CPU Load - Last Month. You can use that as a start for writing your own. Just drop the Average CPU Load from the Select Fields tab, add a new elementary condition on the Filter Results tab for Records where Peak CPU Load is equal to 100, and change the Time Frame to Relative Time Frame, 3 hours.
Hope this helps!