Three.js is just for rendering. Goo Engine is a complete game engine with support for rendering, physics, 3d audio, scripting, state machines, timeline... And there's also Goo Create which makes it super easy even for non-coders to build a scene that can be loaded into the engine.

I'm not sure how to compare extensibility in this case, but both are indeed extensible: For Three.js you might want to add a new class, for Goo you might want to add a new type of component for your entities.