It’s certainly not rubbish to suggest the third placed candidate could win, Eddie. It is a distinct possibility where the three main parties are all quite close and this may be more common if, as proposed, the number of constituencies is reduced (and so each becomes larger).
Certainly the second placed candidate (from round one) has every chance of winning. It comes as no surprise that Ed Miliband is supporting the change to AV because that is precisely how he was elected leader of the Labour Party. His brother David was ahead throughout the contest (having polled 37.78% of first preference votes against 34.33% for Ed). He was still ahead after second preferences had been counted and still in front after third preferences had been taken into account (42.72% to 41.26%). Only when the election had been reduced to the final two candidates and fourth preference votes had been counted did he take the lead (50.65% to 49.35%). So the election to the post of leader of Her Majesty’s Opposition was determined by voters’ fourth choice votes. They had preferred three other candidates before rating Mr Ed, but their fourth choice votes were sufficient to override the votes of those who had ranked David first second or third. “Fair”? I think David Miliband may think otherwise.
There seems to be an attitude among many people that they would sooner see a candidate succeed whom fewest people dislike, or who people like a bit, but not quite as much as somebody else. In elections you often end up with what you don’t want and the more candidates there are the greater will be the number of people likely to be disappointed. It’s an unfortunate side effect of democracy. Fiddling around with the voting system (for that is all that AV is – a fiddle) will not alter that and people need to accept it.