Final answer:
The item in "The Binding of Isaac" that grants a +3 Health Up, with the three heart containers pre-filled with red hearts, and is unlocked by beating the Chest with The Lost is The Body.
Step-by-step explanation:
The item in "The Binding of Isaac" that grants a +3 Health Up, with the three heart containers pre-filled with red hearts, and is unlocked by beating the Chest with The Lost is The Body.
The Body is considered a major upgrade in the game as it increases Isaac's maximum health by 3 red hearts and fills them immediately. This can greatly increase the survivability of the player character, allowing them to withstand more damage before dying.
By unlocking and acquiring The Body, players can take advantage of the additional health to explore more areas, face tougher challenges, and ultimately improve their chances of success in the game.