Why does the Mariner stop " one of three" in other words, that particular Wedding Guest

He has no reason for choosing that particular guest, possibly his is the arm the Mariner grabs because he's the leader of the group, but the mariner is a stranger to them all.

Mariner stops that "one of the three" guest because he was the leader of that group.

 Mariner sopped one of the three wedding guests because he wanted him to listen to his story.