Challenging behaviour of 3yo. I need ideas!

I have a 3 yo who's behaviour has become more challenging of late. I have really noticed the change from toddler to 'little girl'. More defiant, not following instructions, hitting out etc etc. I recently did a mini course on 1-2-3 magic as a way of managing some of these behaviours. Just wondering if anyone has had any success with this method?? Putting a child in time out or quiet time in their bedroom seems to be the method they recommend as a consequence. Even go as far as suggesting to lock the door for short periods if the child continually comes out or until they get the picture. I'm not sure I'm comfortable with this. If she won't stay in her room, does anyone have any ideas??

You could try taking away privileges from her, so she gets the drift that if she does x, then Mum and dad will take her favourite x away, but when she's good, she'll get it back.