When children are angry, they lose cognition. The same happens with adults. (You often hear people say, “I was so angry I couldn’t think straight!” They’re right.) You are dealing with a happy-go-lucky three-year-old one moment, and the next–something sets them off and they become a crying heap of a mess. Therefore, we have to come down to the level they’ve regressed to and allow them to predict consequences for their behavior 100% of the time.
For instance, your 3 year old slaps your 2 year old. Immediately you pluck him out of the situation and say in a loud, firm voice: “You HIT, you SIT.” Immediately he is plopped into a chair, facing the wall, for three minutes. You don’t say anything else, you do not respond to their crying, screaming, pleading, whatever. Sometimes it even helps to have a visual. (Two pictures next to each other: one of hitting, one of sitting, to show the order of what happens.) You may be thinking, What? He knows what hitting and sitting mean! Yes, a three year old does. But an angry or tantruming child loses their reasoning abilities and they are sometimes incapable of listening, processing and understanding words. It helps to have a visual (at eye level) during these situations to point to, and not use any words. At first you may have to assist your 3 year old in sitting in the chair. There is a very safe “hold” called a basket-hold: The child sits in a child-size chair, you are behind the child and cross his or her arms in front of their body, then hold on and either make soothing noises or quietly interject a calming phrase in a whispered voice. (This often works because they stop in mid-cry to figure out what you’re saying.)
Addressing behavior immediately also takes away the social reward of their aggression. They might be hitting/kicking/whatever to a) get the satisfaction of watching their siblings suffer (that sounds terrible, but it’s true most of the time) and b) to get Mommy’s attention, whether or not they realize that is what they’re seeking. If they are immediately whisked from the situation and turned to the wall, they do not get the reinforcement from Mommy or sibling. However…social reward works both ways. When the 3 year old is calming down, even if still in time-out, you praise for calm, good-choice-making, whatever. You do not let an angry, still tantruming child, up from time-out IF THEY ARE NOT CALM. Time-out starts over again and every three minutes he has the opportunity to calm down. (Believe me, after the first few times, the frequency and duration will go down.)
As for jumping off the couch, etc. This is a behavior you might want to replace instead of trying to bring to extinction. For instance, investing in a mini-trampoline. These are relatively inexpensive, fairly safe with supervision and a great energy releaser. You can start telling the 3 year old that he gets one warning about the couch, and then it’s off to time-out. You then remind him that if he wants to jump, he can use the trampoline. The trampoline also becomes a privilege. (As does the tv, certain toys, etc.)
In terms of defiance, “If, first, then” needs to become your favorite phrase. “If you want to play with trucks, first you clean up the playdoh. THEN we play with trucks.” If he screams, whines, whatever, set a timer. “When the timer goes off, I want the playdoh put away. If the timer goes off and the playdoh is still out, Mommy will put it away and there are no trucks today.” Ignore the screaming, crying, begging, whatever after you set the timer. If it becomes out of control after the timer has beeped, time-out again. AND STILL NO TRUCKS!
Re-direct with other activities. “If, first, then” can be used in so many situations…
The only other thing that comes to mind is to praise, praise, praise when it is due. “Good listening!” “I love how gently you help me with the baby.” “That lego castle is so creative.” Etc. Oldests have major displacement issues, especially when the other two are so little and need time. He may have decided that the only way to win your attention is to get negative attention. Turn it around so that he realizes you are constantly paying heed to his good little deeds.