Make a variable for “LastTimeDamaged” (float), then in your damage event, do a Get Game Time In Seconds and set your LastTimeDamaged with it.
Then in your regen functionality, subtract LastTimeDamaged from Get Game Time In Seconds, and check the result - if it’s greater than or equal to 3, start regenerating.
That’s how I’d do it anyway. Delays might be unreliable for this.