When he causes a crit, and deals 3 cards and picks one, does he LOOK at the results of three and choose the desired result or just choose one from the top three from the deck? Just picking blind from the top 3 dosnt seem useful at all.

He reads the 3 cards, chooses one,

and deals it to its opponent.

Repeat the process for every faceup card dealt.

Drawing the top three cards and then "randomly" picking one of them would be COMPLETELY pointless unless the goal is to reduce the damage deck only so those cards that don't get picked get reshuffled in.

If you resolve a [crit] and Marek gets to deal a face-up damage card then he get to take, and look at, the top three cards in the damage and decide which one to hand over.

Worth it to point out, if you do 2 crits with marek, you don't just draw 6 cards and pick 2, you draw three, pick one, then draw another three and pick another one.