Sky Q is the oldest of the three, and uses the traditional set-top box to deliver Sky TV through a satellite dish. Sky Glass and Sky Stream bring you Sky TV over the internet. They have pretty much the same features, but Glass and Stream don't have a hard drive so you can't store recordings locally – instead, you get a Playlist where you can store things to watch later.
Sky Q uses a slightly older user interface, whereas Glass and Stream have the newer Entertainment OS, which look a bit snazzier. While Glass and Stream use the same software, they have different hardware – Glass is built into a TV that you have to buy, whereas Stream uses a streaming 'puck' much like an Amazon Fire TV Stick or Apple TV device.