Food sacrificed due to The Cabbage Merchant's second triggered ability is not being sacrificed to the Food's own ability, so you will not gain any life.
Whenever an opponent casts a noncreature spell, create a Food token. (It's an artifact with ", , Sacrifice this token: You gain 3 life.")
Whenever a creature deals combat damage to you, sacrifice a Food token.
Tap two untapped Foods you control: Add one mana of any color.
,