The number of shell casings found in the Book Depository doesn't prove that only three shots were fired. Based on the available evidence, it cannot be ruled out that there were multiple shooters and more than three shots fired.
Ear witnesses are unreliable.
Garbage in-garbage out.
Question to you Mr Mytton, how many bullets or how much ammo did Oswald own? Are we expected to believe the three bullets he allegedly used on 11/22/63 were the only ammunition he owned? If not, where did he store his bullets?
No one asked me

, but Jack Nessan and his co-author argue pretty persuasively in
Phantom Shot that Oswald fired only two shots and (as I recall) that the dented shell was one Oswald had used for dry firing and had been ejected when he loaded the first live round). A little box from a store in Ft. Worth where Marguerite had once worked was found in Oswald's possession and would have been a perfect fit for a few Carcano rounds. I know from experience that a standard box of 20 high-caliber rounds has always been pretty expensive; my guess would be that he'd never owned more than one box, had used up all the rest in practice with the rifle, and was down to his last 3 or 4 rounds (including the live one found in the rifle) when he shot JFK. The one in the rifle perhaps was saved in case he encountered anyone in exiting the sixth floor.
Because I'm familiar with the acoustics of rifle shots in an environment like Dealey Plaza, I attach little significance to how many shots the earwitnesses thought were fired or where they were coming from. In my own experience, this can be astonishingly misleading.