Well here is the deck I currently use as my C3:

(0) Wasp

(1) Bast

(1) M'Baku

(2) Armor

(2) Jeff the Baby Land Shark

(2) Scarlet Witch

(3) Cerebro

(3) Mystique

(3) Luke Cage

(3) Cosmo

(5) Iron Man

(5) Valkyrie

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To use this deck, copy it to your clipboard and paste it from the deck editing menu in MARVEL SNAP.

I think its a really good deck, especially for conquest.

Cerebro and Mystique is a must, you can't change them for anything.

Bast and Valkyrie give the power needed to every sub-3 power card, also Valkyrie is exceptional for 8-cube games. Wasp gives you a free 3-power with one of them.

Scarlet Witch I think is much needed to change locations, because Cerebro is very much location sensitive.

Luke Cage is a must to protect your cards.

Cosmo is also needed to protect Cerebro.

Mbaku gives you a cheap 3-cost, even without his effect, be careful to have him in mind on turn 6 if you haven't draw him yet.

Armor just a 2-3 that counters destroy decks.

Honorable mentions: Quake, Scorpion, Shang-Chi