Getting off three shots in that time is not hard, but making good shots is, especially if you are not a real skilled shooter, by that, I mean skilled at working the bolt without moving it from the cheek as well as hitting.
Nearly every person will shoot, take the gun from the shouler, work the action, then re aquire a spot weld aim and shoot. This can be done in that time but not with accuracy.
A skilled rifleman will work the action with the rifle still at the cheek. I would bet Oswald was not that skilled.
If there was a second shooter, I would think he was right there or at least next door to Oswald.
The fact is we'll never know. You've made some excellent points and I've enjoyed discussing them. However, I do think there is an element out there ... I'm not really talking about anyone on this thread, so please, nobody take this as a flame ... that if someone came up with a scientifically verified photograph, beyond any reasonable doubt to the point of where it would stand up in every court in the land up to the USSC and even with a big dollop of his DNA on it, of Oswald actually pulling the trigger that day, they still wouldn't accept it.
Oswald had no powder traces on his cheek consistent with having fired an rifle. No evidence of ammo purchases, no extra bullets, no practice with rifle or any indication that it had been sighted after being knocked around going to and from New Orleas. No evidence that he was capable of such accuracy. He was such a crappy shot with a SHOTGUN in the USSR that his friends would give him game after hunting trips.