Does a Thor take 3 or 4 volleys to kill two (2) hydras?

Four

The way multiple attack work is:

  1. the weapon acquires a target
  2. the weapon fires its attacks (all of them)
  3. the animation for the attacks is displayed (usually a projectile, but not in the case of a Thor)
  4. the projectile collides
  5. the damage is applied on a per projectile basis
  6. the unit take decides if it is dead