Óin the Brave

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.

A permanent is any object on the battlefield, including tokens and lands. Spells and emblems aren't permanents.

If your third legendary, Saga, or artifact permanent enters the battlefield and then one of those permanents leaves the battlefield immediately afterwards (most likely due to the "legend rule" or due to being a creature with 0 toughness), you get an enduring story before it leaves the battlefield.