As a teacher of Kindergarten, you may want to slowly increase the hours/days that she is at pre school. It will make the transition to Kindergarten so much easier. You may want to start trying to do special things with the younger two children together as well. I have a feeling that the middle child actually may be more frustrated than the younger one and is taking it out on the younger one by your last post describing her behavior. Your focus needs to be on BOTH of them and assisting them in getting along. You can try the sticker chart for both and role playing with both of your youngest. Fortunately, your oldest seems just fine.
I am here to assist. Just let me know!!!