as someone who now watches other peoples showreels (for sound design more than music), select the choice bits of each project so that it scrolls through a nice range quickly. using the actual footage it was cut to etc is good of course. if you have something you really rate and didn't end up being used, try to cut some stock footage to it or get a filmmaker friend to help with that. doesn't matter if something in the reel was never professionally used or released.
3 mins is usually plenty of time to demonstrate what's possible, but i guess it depends what you're pitching for. the projects I work on tend to be sub-2min so I'm not looking at things that gradually progress in a really cinematic way
as someone who now watches other peoples showreels (for sound design more than music), select the choice bits of each project so that it scrolls through a nice range quickly. using the actual footage it was cut to etc is good of course. if you have something you really rate and didn't end up being used, try to cut some stock footage to it or get a filmmaker friend to help with that. doesn't matter if something in the reel was never professionally used or released.