Is that the number? Changing 3 out of 100 voters' minds doesn't sound to me like enough to get an extra 70 or so seats, but I haven't looked into it and maybe you know better? But I suppose when you consider that perhaps 70 of the 100 won't budge no matter what you do, 3 of the remaining 30 is in any event tougher than it might at first seem.
But it strikes me that, as you know, Einstein's definition of stupidity is doing the same experiment again and again and expecting a different outcome. And you've tried this hard left approach twice now and lost both times. Is trying the same thing again a great idea?
In every walk of life, I think you should evaluate your performances after your victories and your defeats and try to work out why you won or lost, and what you can do to do it better. I don't think patting oneself on the back, blaming others for why you didn't win, and deciding to do exactly the same thing next time is a solid plan. What Corbyn offers is DEEPLY unpalatable to voters such as myself. If Labour was more moderate, personally I am convinced you'd have a much better chance.