.T3D files are files used by the Unreal Engine. Unreal Engine is a program that allows a user with limited programming experience to create multi-player first person shooter games. The .T3D file contains a text list of Unreal map objects, which may also commonly be called map actors. The .T3D file is then used to describe different objects and place them in different locations on an Unreal map. T3D files use different formats depending on the version of the Unreal Engine.