Hello,
as far as T3DGM, I'm not sure. But I think that was created with DarkBASIC Classic, but I could be wrong. If that's the case, this clip from the DBC help file should put you in the right direction:
... If your model file contains texture information, you need to make sure the texture bitmaps are located somewhere within the same directory as your model file. You should also be aware that the X file format only supports bitmap texture filenames in the 8.3 format. This means your texture filenames should be truncated to its first eight characters and end with a '.bmp' extension.
Enjoy your day.