Answer
Use a custom Oracle calendaring expression for the schedule rather than a fixed interval — for example, freq=daily;byhour=8,12,16;byminute=0;bysecond=0; runs the task every day at exactly 8 AM, 12 PM, and 4 PM, and nowhere else. Any valid Oracle Scheduler calendaring expression can be used this way; Oracle's own scheduling documentation covers the full syntax if a different combination of times/days is needed.