Whenever an opponent is dealt noncombat damage, Chandra's Spitfire gets +3/+0 until end of turn.
8/15/2010 Combat damage is the damage that’s dealt automatically by attacking and blocking creatures, even if it’s redirected (by Harm’s Way, perhaps). Noncombat damage is any other damage (generally, it’s damage dealt as the result of a spell or ability).
8/15/2010 The ability triggers just once for each event in which an opponent is dealt noncombat damage, regardless of how much damage that player is dealt. For example, if Lava Axe deals 5 damage to an opponent, Chandra’s Spitfire gets just +3/+0.
8/15/2010 In a multiplayer game, if a source such as Earthquake deals damage to multiple opponents at the same time, the ability will trigger that many times.