I ported this addon for Construct 3, supports runtime 2 and 3.
Included two demo projects - decoding QR code from a static image, and from camera.
https://www.dropbox.com/s/1heq5rf6u8lfjj1/QR_decoder_addon.zip?dl=0
Perhaps you want to add it to the addon exchange, rather than keeping it in your dropbox.
construct.net/en/make-games/addons