Can you deal lethal commander damage to multiple opponents in the same combat step?

In the Commander format, a player loses the game if they have been dealt 21 or more combat damage by the same commander over the course of the game (a format rule, not in the Comprehensive Rules directly, but tracked like a state-based action). This damage is tracked per commander per opponent, so each opponent has their own independent damage total.

During a single combat step, a creature can only be declared as an attacker against one player (or planeswalker) at a time under normal rules (CR 508.1). However, certain effects allow a creature to attack multiple players or deal damage to multiple opponents simultaneously. For example, if your commander has Melee, Trample combined with a blocking setup, or you control effects like Assault Suit (which lets opponents attack with the creature) or Hellkite Charger's extra combat triggers, situations can arise where multiple opponents take combat damage from the same commander in the same turn.

The most common way to hit multiple opponents in one combat step is via abilities that say the creature 'attacks each opponent' or 'attacks each player' such as the ability granted by Goad variants or cards like Avatar of Slaughter. Under CR 508.1, if an effect creates an attack requirement targeting each opponent, the creature is declared attacking each of them simultaneously in one combat, dealing damage to all relevant opponents in a single combat damage step.

Concrete example: Your commander is a 21/21 with an effect that reads 'this creature attacks each opponent each combat if able.' When combat damage is dealt, all three of your opponents each take 21 combat damage from your commander simultaneously in that one combat damage step. Each of them has now received 21 commander damage and each loses the game as a state-based action is checked (CR 704.5).

So yes, it is absolutely possible to eliminate multiple opponents to lethal commander damage in the same combat step, provided the commander can legally deal combat damage to each of them, whether through multi-attack effects, trample-over situations, or similar mechanics.