QIf I play this follower's Fanfare ability when my deck is empty, do I still give it +3 attack?

AYes. You give this follower +3 attack and simply don't put the top 4 cards of your deck into your cemetery. (You don't lose the game either.)

QIf I put the top 4 cards of my deck into my cemetery, and among them were 2 copies of this follower, do I give this follower +6 attack when I repeat its Fanfare ability?

ANo, you still only give it +3 attack.

QIf I put the top 4 cards of my deck into my cemetery, and among them were 2 copies of this follower, do I repeat its Fanfare ability twice?

ANo, you still only repeat it once.

QIs the following correct?

First, I play this follower and resolve its Fanfare ability. I give it +3 attack and put the top 4 cards of my deck into my cemetery. Among those 4 cards was an Electromagical Rhino, so I repeat the Fanfare ability.

For a second time, I give this follower +3 attack and put the top 4 cards of my deck into my cemetery. Among those 4 cards was an Electromagical Rhino, so I repeat the Fanfare ability.

For a third time, I give this follower +3 attack and put the top 4 cards of my deck into my cemetery. None of those 4 cards was an Electromagical Rhino, so I'm finished resolving the Fanfare ability.

AYes, that's correct.