DiRT 3 and its Complete Edition were removed from the Steam marketplace in early 2017. This was likely due to the expiration of licensing agreements for the vehicles and music featured in the game.
In late 2016, the Humble Store briefly offered the DiRT 3 Complete Edition for free to keep forever on Steam. This promotion has since ended.
The resulting .txt file will be saved in your ~/Downloads folder.
If you already own the game and need to generate a technical report for support:
Searching for a " DiRT 3 Free Download" today is complex because the game has been from major digital storefronts. Current Status & Availability