No safe, legal, permanent free Halo 3 PC version exists. Your best bet is buying MCC on sale or waiting for a free weekend.
Here is why:
Discover how to access free versions and community-driven projects of Halo 3 on PC through these detailed video guides: halo 3 descarga para pc free
Dedicated servers with the core Halo 3 feel . No safe, legal, permanent free Halo 3 PC version exists