Actually, it was five rounds, not three. He fought Howard King in 1961, too. And stopped him in two. Liston signed to fight four times in the three years before fighting Ali (or, in this example, Conn). And if all four fights had gone the distance, Sonny would have boxed 50 rounds. But he boxed fewer than five ... because he destroyed his opponents so bad ... the other 45 rounds weren't needed. None of this bodes well for Conn's chances.
Liston was a watershed heavyweight. They didn't build them like him before and his combination of skill, size, athleticism and physical attributes has been the dominating paradigm since. At his best and focussed, he lays waste to pretty much all the heavies that went before him. Woe to the lightheavies that dare tread crossed his path.
I'm curious, since I've seen you say this on Liston a few times. Do you think there are other watershed heavyweights? And who are they?
There is a difference between watershed and paradigmatic. Johnson, Dempsey and Louis were certainly paradigms, with Dempsey being closest to watershed, certainly in terms of status and as a winner of The Event. All who followed him shared something of what he forged in that matter.