Thorin Oakenshield

A single permanent can only count once toward the three legendary, Saga, and/or artifact permanents needed to get an enduring story, even if it has more than one of those qualities. If you control one permanent that is a legendary artifact and a second permanent that is a Saga, you will only have two of the three permanents required to get an enduring story.

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.

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.