I remember the source code for War3ModelEditor being only partially released on purpose as he was afraid people would "steal" it I guess.
Here are some open source projects that do MDX rendering.
Probably the cleanest featureful classic implementation right now, Reforge rendering is not correct though
flowtsohg/mdx-m3-viewer
Kinda messy at the moment, but I know they are working on a big refactor. Does Reforged rendering a bit more correctly I think
Retera/ReterasModelStudio
My World Editor that does classic/Reforged rendering. Both are not entirely correct, but they will do for basic rendering. Check out MDX.cpp, StaticMesh.cpp and SkinnedMesh.cpp
stijnherfst/HiveWE