It's pretty easy as long as you know your polling interval. If you leave it at the default (120 seconds) then 3 intervals is 6 seconds. So your alert trigger is really just:
Node Status is equal to down
and then at the bottom of that same tab, set the trigger delay to 6 minutes (or 7 if you want to be on the safe side).
As a side note, you don't need that "trigger alert when ALL of the following apply" block.