My comment was a bit of an unthoughtful reaction--my bad. I just get tired of pretending that, even in our two-major political party country, we act as if politics isn't about coalitions. The "duopoly" actually consists of multiple groups and interests from various areas of the country. In this respect, the I'll-never-vote-for-the-duopoly-I'm-better-than-you nonsense I see parading around the internet is just lazy thinking at its worst.
At the most basic political government organization level, coalitions can fluctuate too. If the Greens picked up 50 seats in Congress, to have any power, they'd have to caucus with a major party to elect a speaker. If most of these Greens were from the West Coast, especially port areas, they are likely to look at trade a bit differently than Greens in the upper mid-West. Would a Green peel off and vote for Speaker Ryan if Ryan proposed looser trade policies that benefitted port cities on the West Coast? Throwing tariffs on goods from China is likely to effect their districts negatively, after all. If they want to continue to have a role in government, they're in big trouble if they sign onto President Trump's huge tariffs. In this case, we may have "Tri-opoly" in Congress, but with a tense coalition built on trade policy. The outcome would not necessarily be any different than what we have at present in terms of trade policy. It could be worse if the Greens only give conservatives more power (like the Lib Dems in Britain for example).
This happens all the time.