The ~TAG squad, Tomkat, and others have done amazing work on figuring out (most of) the .3di format over years. Tomkat is the only person I know that's imported new models in the game, and he relies on a lot of manual work. I've been working on tools to make it easier.
Just wanted to share what I have so far which is a
.mqo --> .3di converter. The Metasequoia 3D software has a free version, and Tomkat's using it as the source format for his imports.
I haven't gotten textures working yet which is why I used FwO Raven's tool to import them. This only works with simple objects without animations and such. I also hope to make this a .3di plugin in Metasequoia instead of a standalone tool, but it was easier to do my own thing in C# instead of learning C++ for now.
Anyway, if anyone wants to help out or learn more, hit me up.