Shooting arrows in volleys is fairly common in reenactments, so you can totally get a bunch of archers to do a synchronized "nock, draw, loose" with a bit of practice. I suspect, however, that this is done mostly out of modern safety concerns that a battlefield commander wouldn't care about so much.
If you give the commands as "nock!" and then "draw and loose!" rather than "nock, draw... loose!" it's even easier, though you won't get the same level of synchronization. I could imagine this being used at very long ranges when shooting to provoke rather than "for effect," to borrow a modern term.