The cron expression for every 3 minutes
*/3 * * * *
The minute step */3 resolves to 0, 3, 6, 9, …, 57 — 20 firings per hour. Three is a divisor of 60, so the cadence is perfectly even (no 4-vs-3-minute alternation that you get with non-divisors like 7).
See the dedicated guide for the full breakdown: Cron every minute.
Paste it into the editor above to see the next runs on a calendar.