Model 11's never had a magazine cutoff, the MC only moves a little piece of metal to physically block off the magazine. I also don't think that Browning ever made a 3 shot A5. Put a little bear grease or 40 weight motor oil on the magazine tube where the bronze bushing is. My model 11 has never failed me in the field and 2 years ago I used it in - 27F weather, pheasant hunting. You won't find a more reliable gun in my opinion.
Edit - When you said tried the ring at the rear, do you mean you put the bronze bushing below the spring or are you talking about the steel ring that is flat on one side and concave on the other?