My 3.5 year old boy has responded very well to the card system! 3 cards lost and spend the rest of the day in your room. He also knows that hitting/kicking a person /the wall would be game over immediately. So the only behavior we have used the cards for is insubordination. Sometimes when I tell him what to do, he will growl or roll his eyes and sigh but he does do you whatever we tell him.
However, he will test the limits. For instance, he got mad at his baby sister and slammed a toy down really close to her. I yelled at him and told him to move away from her immediately. But I didn’t take a card or send him to his room.
Another problem is how to tell him how to be gentler with his sister. I will tell him not to do a certain behavior but then he will do a similar but equally rough behavior.