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Step-1) First access Xfire's Options from the Tools menu.
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Step-2) Change to Games tab and double click on Not Installed
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Step-3) Find the game you want to add, select
Manual Setup and browse for the installation folder, for Liberty Unleashed you need to select gta3.exe.
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Step-4) Press Apply and the game is now recognized by Xfire.
Here is the complete tutorial about it.