My guess would be that he doesn't know how to express his feelings about what is going on. A three year old can talk and reason, but this is too much for him, as I'm sure he doesn't understand what is happening. If it was me, I'd constantly reassure him that you both love him and siblings and the only thing changing is you 2 won't be living in the same home any more. I'd talk to him about his behavior and let him know it's unacceptable, but do realize it's his only way to communicate about changes taking place. Unless he's very advanced for his age he probably doesn't have the words to express his feelings.

Hug him often and reassure him. Give him a hug from me