The way of choosing the number of active reels is choosing the slot game to have the desired amount you want. If you want 3 reels then play a three reeled slot game or if you want to play 10 reels then play a ten reeled slot game.
Trying to change the number of active reels on an existing slot reel design is not allowed as they are set to be a 4 reel slot or 5 reel slots.