Well I'm not sure if this is within the scope of the program and I'm not sure if I can help you. I don't have any knowledge on Three.JS and the .model files are normally used for just saving and loading Asset Forge models. They aren't meant to be an output type, however if you can make it work I'm all for it.
Do keep in mind that the .model file format might change in a future version of Asset Forge, so not sure if it's the best to work with that.