Thirty five yards is about my limit. Fifteen-twenty yards for a good percentage one shot kill.
I've actually had poor results with any kind of Buck shot, though I haven't tried 3 Buck. I get better results with 2 shot (Goose loads) or BB, magnum or 2 3/4 semi magnum.
Just my experience. I'd expect better results with 00 Buck, but it has *never* turned out that way for me.