Prevent the next 3 damage that would be dealt to any target this turn by a source of your choice. You gain 3 life.
Healing Grace only targets the creature, planeswalker, or player that will receive the damage prevention “shield.” The source isn’t targeted.
You choose only one source, even if that source won’t deal 3 damage.
You gain 3 life as Healing Grace resolves, even if there’s no damage to prevent.