Rename Asteroid.prefab to Asteroid1.prefab
Add Asteroid2 to Scene and give it a rigidbody
Add Collider, Scripts, fx to Asteroid2, turn it into prefab

* Capsule Collider, not a trigger
* RandomRotator.cs, DestroyByContact.cs, Mover.cs (Same as Asteroid1)
* Turn it 180 degrees around Y-axis so it moves in the right direction
* Give it same speed and tumble as Asteroid1
* All explosion effect
Add Asteroid3 prefab, same treatment as 1 and 2
Spawn all asteroids with equal chance

* Speed in Mover scrips changed to negative to compensate for
  Asteroids2&3 facing the other way
* Correct the hierarchy of Asteroids2&3. Now props are child objects
  of a new GameObject with scripts, rigidbody, collider attached.
  This is just like in Asteroid1.
Add missing Asteroid models for asteroids 2&3