The Lucky Charm is an item found only in Mario Party 3. It cannot be bought in an item shop or won in an item minigame. It is only obtained by buying an Item Bag at an item shop or meeting Toad on an Item Space.
When used, Game Guy appears and forces the target to risk all of their coins on a chance game, one of Game Guy's Minigames. If it's won, he will double the player's coins; if it's lost the game he will take all coins. If a player owns a Lucky Charm, he or she could select the player which Game Guy will manage, or even choosing oneself.