Underworld Cerberus can't be blocked except by three or more creatures.
Cards in graveyards can't be the targets of spells or abilities.
When Underworld Cerberus dies, exile it and each player returns all creature cards from their graveyard to their hand.
9/15/2013 If you can’t exile Underworld Cerberus when its last ability resolves, perhaps because it’s been exiled by another spell or ability, each player will still return all creature cards from their graveyard to their hand.
9/15/2013 If two of these die at the same time, the last ability of each will trigger. If each player controlled one, the nonactive player’s ability will resolve first. The Underworld Cerberus controlled by that player will be exiled, and all other creature cards in graveyards (including the Underworld Cerberus controlled by the active player) will be returned to their owners’ hands. Then the active player’s ability will resolve. That Underworld Cerberus won’t be exiled, but any creature cards in graveyards at that time, perhaps because they died in response to this ability, will be returned to their owners’ hands.