Halo 3 Campaign Max 2 Players Offline — No 3+ Player Co-op Until Halo Infinite (Online Only)

No. Halo 3 campaign supports a maximum of 2 players via split-screen, both online and offline. There is no 3- or 4-player campaign co-op in any Halo game until Halo 5 (which dropped split-screen entirely) and Halo Infinite (4-player online co-op only, no local).

For 4-player local Halo, you need System Link (4 consoles × 2 players each = 8 total) in Halo 3, Reach, or Halo 4.