If you don't want to slow down your brower with add-ons, you may download the stand-alone (and free!) YouTube Downloader HD. The last version knwon today (version 2.9.9) weights only 9 Mbytes. Usable on any Windows 2000/XP/2003/Vista/7/8/10. You may save the downloaded video in .FLV, .AVI, or .MP4 formats. Resolution settings are offered; in most cases, it is wise to accept the largest resolution.

In case your 3D screen is broken, or if you are blind, downloading audio only from a 3D movie make sense. It is easy thanks to the YouTube to MP3 converter from the same author as the YouTube Downloader HD here above.