Incorrect file associations could be the result of underlying issues within your PC system. As such it is highly recommended to scan your PC for invalid registry entries, unused processes and other unfavourable system settings to identify slowdown issues.
The UT3 file type is primarily associated with 'Unreal Tournament 3' by Epic Games, Inc..
Unreal Tournament 3 (UT3) is a first-person shooter and online multiplayer video game by Epic Games and is the next installment of the Unreal series after Unreal Tournament 2004.
It is published by Midway Games and was released for Microsoft Windows on November 19, 2007, PlayStation 3 on December 11, 2007, and on Xbox 360 on July 3, 2008.