I think I assumed NPCs would fight until three successful strikes occurred, whether by them or against them. (The “Anyone who reaches 0 HP…” rule would then apply to the PCs.)
If you need something more concrete, I would recommend either giving everyone something like d6 HP or HP based on the tournament round and the “three successful strikes” rule (which would make later rounds more difficult for the PCs, I recognize)–so something like:
Round 1: 3 HP Round 2: 6 HP Round 3: 9 HP