Final answer:
The item in The Binding of Isaac that offers 'HP up x3' and other benefits, and is exclusively found in the secret room pool, is the Black Lotus.
Step-by-step explanation:
The item in The Binding of Isaac game that provides the attributes you've described ("HP up x3," one Health Up, one Soul Heart, and one Black Heart, with depleted Black Hearts causing 40 damage to the entire room) and is found exclusively in the secret room pool is A) Black Lotus.
This item effectively increases the player's survivability by adding to their health. When a Black Heart is lost, it causes damage to all enemies in the room, which can be extremely helpful in clearing rooms full of enemies or dealing with tough situations.