The official version of Halo 3 on PC is part of (MCC).
A fan-made mod that brings a version of Halo 3’s engine and multiplayer to PC for free. Halo 3 Free Download PC Setup
Finding an official "free download" for on PC is difficult because the game is primarily a paid title. While there are no legitimate, permanent free versions of the full game, there are official ways to play at low cost and community-driven alternatives. Official Ways to Play The official version of Halo 3 on PC is part of (MCC)
This version is strictly multiplayer-focused and does not include the official Halo 3 campaign. PC Setup & System Requirements While there are no legitimate, permanent free versions
Includes full keyboard and mouse support, FOV sliders, and a server browser for custom games.
You can play Halo 3 for "free" with an Xbox Game Pass for PC subscription.