The I/O boards are the same, you can just leave them there. The 5 line "display" is nothing more than a light box with 5 either leds or incandescent bulbs, not entirely necessary. Depending on the backlight controller you have, you may or may not need one. Do you have the led controller, or the older style incandescent controller? You can use the 14 button deck on 3 reel games, the only buttons that work would be Max Bet, cash out and Service, but you can play the game. So in short, the bare minimum is:
3 reel shelf with reel harness for backlit reels.
3 backlit reels
To make it look "authentic" you will also need
5 button deck (specify your reel glass type, as the harnesses are different if you have a 6-6-3 display)
5 line stepper light.