If Darius is the third card played in the turn would he be readied?

No, if Darius, Trifarian is played as the third card in a turn, he will not be readied by his ability.

According to the rulings in FAQ #6266 and FAQ #3905:

• Darius, Trifarian can only count cards played in the turn that he is able to "see," which includes cards played after he resolves.
• Because he is played as the third card, he did not see the second card being played. Therefore, his trigger condition—which requires playing a second card in the turn—cannot be fulfilled by playing additional cards after him.

Key Nuances:

• Darius, Trifarian must be played as the second card of the turn to trigger his ability. When played as the second card, he "sees" himself as that second card and will ready himself (as confirmed in FAQ #6776 and FAQ #1119).
• Playing him as the third, fourth, or any subsequent card will not trigger the ability, as the condition of playing the "second card" has already passed.

This interpretation is based on the trigger condition of his ability and the established rulings regarding his ability to "see" cards played during the turn.

Note: I could not fully verify the exact phrasing of the trigger condition against the retrieved rules.