Bifur, Melodic Rider

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.

Once you have an enduring story, you have it for the rest of the game, even if you lose control of some or all of your storied permanents. The enduring story designation is on the player and can't be removed by any effect.

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.

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.

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

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.

Bifur's last ability doesn't copy the triggered ability; it just causes the ability to trigger an additional time. Any choices made as you put the ability onto the stack, such as modes and targets, are made separately for each instance of the ability. Any choices made on resolution, such as whether to put counters on a permanent, are also made individually.

Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities are triggered abilities and will have "when," "whenever," or "at" in their reminder text.

In some cases involving linked abilities, an ability requires information about "the exiled card." When this happens, the ability gets multiple answers. If these answers are being used to determine the value of a variable, the sum is used.

If a triggered ability is linked to a second ability, additional instances of that triggered ability are also linked to that second ability. If the second ability refers to "the exiled card," it refers to all cards exiled by instances of the triggered ability.

If Bifur entering causes you to have an enduring story, his "enters or attacks" ability triggers an additional time.