Well, Donahue mentions the errant shot that missed and hit the road, then the shot that hit Kennedy in the back.
His evidence of the third shot being from the SS agent and the subsequent bullet fragments in the skull have not been disputed (the fragments in the skull).
I'm not privy to any information relevant to a third shot supposedly fired by Oswald..or what was the evidence that supported it.
That was the first shot which was fired late, when the vehicle appeared into view, surprising Oswald. That shot struck the blacktop behind the vehicle and richocetted, breaking apart, raining bullet fragments in the direction of vehicle. Kennedy was heard to say that he'd been shot, when he most likely was feeling the impact of the bullet fragments in the back of his head. Some of this jacket material struck the inside of the windshield, damaging it (I think Donahue may have photo). Some fragments were found on the floor in front of the front seats.
His evidence of the third shot being from the SS agent and the subsequent bullet fragments in the skull have not been disputed (the fragments in the skull).
I do not believe this is correct. The bullet fragments are most likely from Oswards first shot, as described above. Bullets which strike flesh leave their jacket material within, not on the surface, either at the point of entry or exit.
I'm not privy to any information relevant to a third shot supposedly fired by Oswald..or what was the evidence that supported it.
Donahue argues that the third shot was fired by SSA Hickey in the trailing limo when he instictively clutched his fingers firing the weapon while the limo lurched haltingly at some point after the first two shots.
The book "Mortal Error" by Bonar Menninger documents Donahue's investigative work and subsequent effort to get his theory heard. I do not know if it still in print. It might be available used through amazon. Worth checking out.
If you want a coherent explication of Donahue's theory, then you should read the book.