When a spell or ability instructs each player to discard a card, starting with the player whose turn it is and proceeding in turn order, each player selects a card from their hand without revealing it, sets it aside, and then all of those cards are revealed and discarded at once.
If an opponent discards a card that shares two card types with the card you discarded, such as an artifact creature, that player still only loses 3 life.
The card types that may be shared for Creeping Dread's ability are artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Supertypes (such as legendary and basic) and subtypes (such as Human and Equipment) are not counted.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Anthony Palumbo
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If a player has no cards in hand, that player doesn't discard a card while everyone else does. If you didn't discard a card, no player can have discarded a card that shares a card type with the card you discarded, so no player loses 3 life.
Because you consider only the characteristics of a double-faced card's front face while it's not on the battlefield, a double-faced card with different card types among its faces (such as Skin Invasion) only compares its front face to the other discarded cards.
