There is only one file type that uses the .P3T extension:
PlayStation 3 Theme File
Theme file for a Sony PlayStation 3 video game system; includes background images, buttons, layouts, sound files, and other settings used to customize the interface of the PS3; can be downloaded and installed using the PS3 console's browser application.
P3T files can be transferred to the PS3 using a Memory Stick or USB connection. They should be placed in the :/PS3/THEME directory. If the THEME folder does not exist, you must first create the folder, then transfer the P3T file.
Updated: September 22, 2014