I don't think he was a 3 rounder. Stamina issues, for sure. He walked through some fire from way bigger hitters than Usyk.
Usyk for me stops him. Sander's was dangerous at what he did, sneaky counter puncher with fast hands but Usyk is defensively a lot better than Wlad back then, Sanders may never land his left hand and even if he does, Usyk's shown a chin that can take serious power. At best Sander's lands but Usyk survives and in the 2nd half turns the screw and Sander's poor conditioning is exposed and Usyk breaks him down and stops him. More likely he never gets to land the left hand at all and is just dominated. Usyk isn't going to go right at Sanders like Wlad did and give him opportunities to land the counter. Usyk's tendency to start slow and build into a fight helps him here when Sanders is dangerous early.
I'm a huge Sanders fan but this one is tricky for him. TBH I'd prefer an earlier version of him that perhaps did not hit so hard but which had more speed, stamina and mobility. Usyk has great footwork which is the bane of all the less mobile heavyweights that he has faced. He also has unpredictable upper body motion and is great at disrupting rhythm and at finding the gaps in an opponent's game. Corrie's equalizer is his excellence in a narrow subset of abilities at that age. He always had good accuracy, great speed and great power. Darting in and out and moving on a guy is well and good but you also need to do that and not get hit. Sanders is faster and more accurate than anybody Usyk has faced at heavy and probably equal to AJ in punching power. Sanders managed to land on almost everybody he faced and usually the guys needed a great chin and / or conditioning to make it through. Unfortunately Usyk has both. So to be brutally honest, Usyk probably takes this because he is simply the more dedicated, well conditioned and unusual fighter. I'm not writing Sanders off, though, because the dude had the touch of sleep with the right punch, and he often landed it.
Sanders wasn't a wrecking ball, he was a counter puncher. Wlad was an aggressive fighter back then and got timed, and his chin couldn't take the shot. Usyk would never give Sanders that opening. Usyk stops him.
I think Corrie has a chance. Usyk likes to walk people down. That's very dangerous against a massive puncher with nice speed and timing. Usyk isn't stopping Sanders if he wins. Usyk isn't featherfisted but he's not stopping a guy like Sanders who had a great chin.
Sanders was dangerous the first handful of rounds. Usyk is skilled enough to outlast past that point and win.
Mistakes orthodox fighters make against southpaws is coming in too squared on, he was able to capitalise on that, but his main weapon wouldn’t be nearly as effective against a fellow southpaw and he doesn’t really have any other tools that are elite, and Usyk isn’t exactly predictable in his attacks either, Usyk stops him.
Usyk would beat Sanders easily. There might be a scare or two, but Usyk would just get on his bicycle and wear Corrie down, then probably stop him mid or late.