Prove that if an object at rest explodes into exactly THREE fragments, the velocity vectors of the three fragments always lie in a PLANE- no matter...

Prove that if an object at rest explodes into exactly THREE fragments, the velocity vectors of the three fragments always lie in a PLANE- no matter what the masses of the fragments! Give a general proof, not just a random numerical example. (HINT: think of a clever choice for what to call the x-y plane.)