The cron expression for every 3 hours
0 */3 * * *
The hour step */3 selects 0, 3, 6, 9, 12, 15, 18, 21 — eight firings per day. Three is a divisor of 24 so the spacing is perfectly even.
See the dedicated guide for the full breakdown: Cron every hour.
Paste it into the editor above to see the next runs on a calendar.