Incapable of making a fist, an open palm or a peace sign? Try this electronic Rock Paper Scissors gadget. For $8.89, you get two so you play RPS with a buddy with minimal (and we mean minimal) effort.
If your problem is your inability to pull the trigger and choose rock, paper or scissors after the three-count is done, this should alleviate your problems. But if you've got trouble doing anything at all on time, then even pressing the button on three will be too much. However, there's no way to screw with your opponent and throw five rocks in a row just for the hell of it.