So if Fenn uses the DDC and activates havoc shot, would it register for all three attacks while taking just 1 stress?

You don't activate Havoc Shot when you declare your attack, you activate while you attack. Meaning that you will never, ever use Havoc Shot unless you know for a fact that you will deal damage to trigger blast.

"Special Situations Regarding Attacks", RRG, Page 6:

Any ability that is resolved “while attacking” or “while defending” can be used at any point during the attack with the following exceptions:

-- An ability that adds dice to a dice pool can only be used immediately before step 2 of the attack.

-- An ability that allows a player to reroll dice can only be used during step 3 of the attack.

-- An ability that allows a player to modify die results can only be used during step 4 of the attack.

That said, each attack is resolved separately, so you would have to use Havoc Shot up to three times (suffering 1 strain each use of Havoc Shot).

Next time, please put questions like this in the "Rules Questions" sub-forum.

Thanks!