D.R.
I can see how this would get so frustrating.
#1 He could be acting out from the change of the move. With kids it seems like changes that we cope with they can't cope with and his way could be acting out...he gets attention that way.
#2 I would also limit Red Dye #40...studies have linked it to ADHD. A LOT of food have Red Dye #40 so its impossible to totally wipe it out of your diet, but you can limit it. That has been a life-saver in my household.
#3 Is he catching violence on TV? I had a child in my care that watched Peter Pan (yeah yeah its Disney, but Captain Hook fights the crocadile in it) and started wanting to fight with the other kids. You would think that it wouldn't be something as that...but we stopped watching Peter Pan and its been a lot better.
#4 Is he getting enough exercise and other activites to occupy his time. He might need more physical activity (running in a field, kicking a ball) to get all this extra energy out that he is trying to get out negatively now.
You could try a behavior chart or jar. Figure out something that would work with the kids....You could use a baby food jar and fill it up with something when the kids are good, take it out when the kids are bad....if they get it full, they get a small reward.
Both kids should be able to comprehend that it is not ok to fight. I would keep trying TIME OUTs...for me, I don't start the time until the child is quiet and sitting still. 3 year olds are very strongwilled and independent and do not want to listen. What has worked with my daughter is that I can take away her favorite toy (she wouldn't get it back until the next day or in some cases for a couple days)