The claim that Instant Runoff Voting helps third parties is false. In fact, we know from studying the countries and jurisdictions that use it, IRV entrenches the two party duopoly and prevents weak third parties from gaining strength. The only time IRV does not harm third parties if if they are strong to begin with.
IRV leads to two party domination:
"The three IRV countries: Ireland (mandated in their 1937 constitution), Australia and Malta (and more recently Fiji for a brief period of IRV democracy before its coup) all are 2-party dominated (in IRV seats) – despite having many other features in their governments which would seem much more multiparty-genic than the USA with IRV added will ever have. So you can be sure the USA with IRV would be 2-party dominated too." - from the Center for Range Voting's report "Why does IRV lead to 2-party domination?
Why Two Party System is entrenched wherever IRV tried:
"IRV has entrenched the two-party political system wherever it has been tried. xiv One reason is because if a voter puts a third party candidate as his or her first choice, it can hurt the chances of the voter’s second choice major party candidate, who could potentially be eliminated in the first round, causing that voter’s last choice to be selected for office.xxvExamples include Australia (IRV seats are two-party dominated, zero third party members currently in the federal house; even though other NON-IRV seats NOT 2-party dominated, so this makes it quite clear) ditto Ireland and Fiji (but Fiji's democracy recently ended)."
From From Kathy Dopp's report "Realities Mar Instant Runoff Voting -18 Flaws and 4 Benefits"
More on IRV and Two Party Rule:
Australian Politics - the "Disadvantages of the Preferential [IRV] System"... promotes a two-party system to the detriment of minor parties and independents.
Libertarian Reform Caucus "Anyone for a Bullet in the Foot? Instant Runoff!"
There are other election methods that help third parties without the flaws that IRV suffers from.