Press the "3D Screenshot" shortcut key when you are running the game. By default it is set to PrtScn/SysRq key. Additionally, open the TriDef 3D Ignition OSD and select 3D Screenshot in the menu.
3D screenshots are stored in the current user's picture folder. When the 3D screenshot is successful, there will be a message in the lower left corner of the bottom indicating the location of the file storage. Each 3D screenshot is stored in several format folders.