This is the location of the addon folder if you are using Blender 2.79

https://lgtm.com/projects/g/mrdoob/three.js/snapshot/9787df2d17f268a60d2b194a532620a8ee502111/files/utils/converters/fbx/convert_to_threejs.py?sort=name&dir=ASC&mode=heatmap&excluded=false#xaaf74a80914aab53:1

If an animation key doesn't explicity define the scale value, it is set to (1, 1, 1) instead of (0, 0, 0).

This assignment to 'FLOAT_32' is unnecessary as it is redefined here before this value is used.
=>
https://lgtm.com/projects/g/mrdoob/three.js/snapshot/109bbfaeab1a0768acf40151648acfdf37a1949f/files/utils/exporters/blender/addons/io_three/constants.py?sort=entity-kind&dir=ASC

- Export materials before meshes, so they can be applied to faces.
- Raise an error on faces with > 4 vertices instead of writing JSON
  which will fail to load.
- Correctly export transparency -- black in Maya means opaque.

The path for addons is different for 2.76, after searching my filesystem
this is the location I found for my addons directory