Dáin, Lord of the Iron Hills

Storied isn't a triggered ability and doesn't use the stack. Players can respond to a spell that will give you your third legendary, Saga, and/or artifact permanent, but they can't respond to you getting an enduring story once you control that third permanent.

While the effect of Dáin's last ability is applying, your opponents can choose not to attack with a creature that must attack if able as long as there is no other player, planeswalker, or battle for that creature to attack that wouldn't require a cost.

If you control three legendary, Saga, and/or artifact permanents but don't control a permanent with storied, you don't get an enduring story. For example, if you controlled three artifacts, lost control of two, then cast Thorin Oakenshield, you won't have an enduring story.