QDoes this follower's ability mean that it can only take 3 damage a turn?

ANo, it means that any instance of damage dealt to this follower will be reduced to 3 if it exceeds 3 damage. For example, if this follower is attacked by two enemy followers with 4 attack, it will only take 3 damage from each attack. However, if this follower is attacked by an enemy follower with 4 attack and an enemy follower with 2 attack, it will take 3 damage and 2 damage from each attack respectively.