Yes Newton's Third Law applies. What would happen according to that law if a large amount of matter was thrown forward from a head wound?
As has been asked repeatedly of the "shot came from the front" crowd, what kind of bullet causes a body to jerk back the way JFK's did? The ballistics experts I've read all said there is no such bullet. It makes sense if you apply Newton's law. A bullet doesn't have the power or force to drive a body back like that.
What was JFK hit with, a cannon ball? This wasn't a movie scene where the bodies go flying after being hit. That's not what the experts say a body does when it's hit.
So, if the bullet didn't - and can't - cause such a violent reaction then something else did.