Had some success with this, some programmer friends took a look for me and within a day made a script to convert .3db files into a usable .gltf format
Now I should easily be able to make new textures for the models! Maybe people would find it interesting if I started a thread here on the forum for the texture project.
I've put up the scripts on GitHub for anyone that's interested https://github.com/djunkles/Diggles3db
The older scripts found in this thread are also in there (hopefully that's okay) as they were used as reference to make the converter tool.
There is still some work to do, as it currently doesn't extract animations and only extracts the first object in a file. Also it would be nice for the end product to have an actual interface.