If a player is dealt lethal damage from multiple sources in the same combat, does the order of damage matter for triggers?

During the combat damage step, all combat damage from all attacking and blocking creatures is assigned and then dealt simultaneously (CR 510.2). Because all the damage happens at the same moment, no single source 'goes first' — the game does not process damage source-by-source in sequence.

State-based actions (like a player losing the game for having 0 or less life) are checked only after all the damage has been dealt and after any triggered abilities that resulted from the damage are placed on the stack (CR 704.3). So even if three different creatures each deal 3 damage to a player at 7 life, the game sees all 9 damage at once, then checks state-based actions once.

Triggered abilities that say 'whenever this creature deals combat damage' do all trigger at the same time. They are then placed on the stack in active player/non-active player (APNAP) order, and their controller chooses the order among their own triggers (CR 603.3b). The order they resolve can matter for their individual effects, but it does not change the fact that the player was dealt lethal damage — that outcome is already locked in.

Concrete example: An attacking Vampire Nighthawk (deathtouch, lifelink) and a Grizzly Bears both deal combat damage to a player at 4 life simultaneously. The Nighthawk deals 2 and the Bears deal 2 — the player goes to 0. Both 'whenever this creature deals combat damage' triggers (if any) trigger at the same moment. It doesn't matter which trigger resolves first; the player is already at a lethal life total as determined by state-based actions after all damage was dealt.

In short, simultaneous combat damage means there is no meaningful 'order' among those damage events for the purpose of triggers or determining lethal damage — everything happens in one game action (CR 510.2, 704.3, 603.3b).