Pain Magnification

• WWizards of the Coast2006-05-01
  Official Ruling
  If a source deals 3 or more damage to multiple opponents at the same time, this ability will trigger once for each of those opponents. They will each have to discard a card.
• WWizards of the Coast2006-05-01
  Official Ruling
  This triggers only if all 3 or more damage is dealt at the same time. It doesn’t trigger if damage that adds up to 3 or more is dealt at different times by the same source.

Rulings via Scryfall