Personally, I prefer the term threenager. Sums up the "attitude" that some seem to develop at 3.

When this type of conflict comes up, I have the kids involved come to the kitchen table and sit to "work it out." The first few times, I will facilitate the interactions, but basically, I want them sitting, and talking to each other. They aren't to tell me what they want, or what happened, they have to work it out between themselves. "Tell her why you are upset." "Decide between the 2 of you who will play with the toy." When they are sitting at the table, they are looking at each other but can't really get physical because there is a table in between them. I leave it up to them to decide when they are ready to go back to play, because it isn't a time out. If they go right back to fighting, then they come right back to the table. After awhile of this, they will do it on their own. And I will only intervene when there is a lie, or someone says something unkind or hurtful. "I'm not your friend anymore." is not allowed.