Do you mean that whole grow space is 3 foot tall max, or you have about 5 feet and are using a HPS? If your area is 3 foot tall then im sure you using CFL or LED. CFL tend to make plants grow bushy if you keep them nice and close. Also tieing branches down and tieing the main stalk down will help you keep your plant shorter. Choose a indica dominant strain for all around shorter bushier plants. Your plant will at least double in size when you start to flower, so plan accordingly. You could always go 12/12 from start to make sure you have enough room.