You can buy Halo 3 individually on Steam for roughly $9.99 , or even less during seasonal sales.
Occasionally, the collection is featured in Steam Free Weekends , allowing you to download and play the full game for a limited time at no cost. Community Alternative: ElDewrito (Halo Online) Halo 3 Free Download PC Setup
This version is strictly multiplayer-focused and does not include the official Halo 3 campaign. PC Setup & System Requirements You can buy Halo 3 individually on Steam for roughly $9
Includes full keyboard and mouse support, FOV sliders, and a server browser for custom games. PC Setup & System Requirements Includes full keyboard
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