Cron every 3 hours: 0 */3 * * *

Three divides 24 into eight equal pieces, so this is one of the few multi hour schedules with no awkward remainder. The gap from 21:00 to the next run at 00:00 is three hours, exactly like every other gap, which means a job that assumes uniform spacing between runs is safe here in a way it is not with a step of five or seven.

Eight runs a day is a common choice for syncing with a partner system that publishes on an unpredictable cadence. It is frequent enough that nothing sits stale for long, and infrequent enough that each run can afford to do real work: a full comparison rather than an incremental one, which removes a whole class of drift bugs.

This schedule is also the backbone of the two line trick for a 90 minute cycle, because 90 minutes and three hours share a period. If you find yourself writing 0 */3 alongside 30 1-23/3, that is what is going on, and the pair should be commented so the next reader does not delete one of them.

The first run of the day is at midnight, which puts it in the same slot as nightly maintenance on most systems. Shifting the whole set with 0 1-23/3 keeps the three hour spacing and moves every run one hour later, which is usually enough to get out of the way of backups without changing anything else.