If you choose a target creature card in your graveyard and that card is an illegal target by the time the triggered ability tries to resolve, the ability won't resolve. You won't gain 3 life. If you don't choose a target at all, you just gain 3 life.

The triggered ability can target a creature card that was put into your graveyard as a result of state-based actions immediately after the combat damage that caused the ability to trigger.