Oh man I love watching this guy's totally entertaining videos. I'm not even a three.js developer (yet!) and I still watch them now and then. Somehow affirming of life...
The weapon models are actually the stock Quake II models/textures except in the case of the first one, which is the model included with the custom mesh. These are technically freely available through the Quake II CTF demo, but fair warning if you attempt to use these in your project.