If I play Stacked Deck as my first card for the turn, then play Darius Trifarian as my 2nd card for the turn and then play a Rebuke for my 3rd card for the turn, does Darius Trifarian activate?

Yes, Darius, Trifarian will activate.

Here's the sequence:

1. Stacked Deck resolves as your 1st card.
2. Darius, Trifarian is played as your 2nd card. He enters the board and his triggered ability sees that the second card this turn has been played — because he is that card.
3. His ability triggers: +2 Might this turn and he readies himself.
4. You then play Rebuke as your 3rd card, which has no effect on the already-triggered ability.

Per FAQ rulings: "Darius counts himself as a played card. If he is your second card played this turn, his ability will trigger." (RiftJudge FAQ #9135). The fact that a third card comes after doesn't undo the trigger — the condition was satisfied the moment he entered the board as the second card.