Darius, Trifarian

Q: If I play Darius as my third card, does he ready and gain 2 Might? What if I play another card after him?

No. If Darius isn’t present immediately after you play your second card, he won’t trigger that turn.

Q: If I play Darius, Trifarian as my second card in a turn, does he ready and gain 2 Might?

Yes, this works. Triggers like this are evaluated immediately after the event that takes place, at which point Darius is on the board.