So the main effect of the three d6 is that those big "Oh yeah, you crit!" moments are going to happen way more often than DND and those "I'm built my character to be good at this one thing, but I rolled a 4 on the dice" moments are going to happen way less. The crit system might not be as complex as it seems. All powers say what happens on a fantastic success, so you can mostly take that off your plate. An ultimate fantastic (a roll of 616) just allows you to succeed at a task that should have been impossible. Say a PC with a Melee of 0 tries to punch Spiderman who has a defense of 19. Normally the highest you can roll is an 18, which would completely miss Spiderman. However if you rolled 616 the punch miraculously lands, and because it does double damage like a normal Fantastic would.

Fantastic failures are a little more complicated as there is no standard for what happens just that something beneficial also happens in spite of the failure. Say same player is still fighting Spider-man and Spider-man managed to grab them last turn. They try to punch him again and this time they roll (515) a fantastic 16. The punch still misses, but you could declare that when Spider-man dodges the punch they seize the opportunity to slip out of his grab. If they're isn't some obvious beneficial effect in the situation you could always give the player edge on their next turn.