Borderless: In-game, save-point. Close. Edit %USERPROFILE%\Documents\ My Games\DiRT3\hardwaresettings\ hardware_settings_config.xml. fullscreen="false". ShiftWindow (+ border = borderless). OR Fullscreen: SoftTH (has proof, untested) & enforce DX9 (probably make forcedx9 true in file %USERPROFILE%\My Documents\??\hardware_settings_config.xml).
(PLP screenshot with bezel effect)
View guide for general help with the instructions.