Create an event with these conditions:
(secondsLeft = 0) and (roundNum <= 3)
Add actions to pause the clock and whatever else you want it to do. Make sure that you move this new event above any others that may fire at the same time, like an end-of-round alarm (I made this mistake in trying this).
I came across an interesting bug, I think, in trying this: when the clock pauses at 0 seconds left, it actually goes back to 1 second left. And this gets it stuck in a perpetual loop because when you unpause the clock, it hits 0, pauses automatically because of the event, and rolls back to 1 second left... See if you see the same thing.