Because she'd just seen he husband's head explode while she was staring straight into his eyes from almost nose to nose distance?
I have a Mannlicher Caranco 91/38, not a bad little rifle, but I can't see anyone using a scope could get off three rounds at a moving target.
I do not have one, so I cannot test how quickly one could shoot it. But remember that the first shot would already have chambered.
I do not know how well Oswald's Marine indoctrination went. He fit the classic loner-chip-on-his-shoulder mentality, so I doubt he was a good Marine.
I think she was trying to pick up pieces of his head? Kind of tells me the point of impact was from the front.