Will there ever be a viable third party in the United States?

Possibly (it's happened before -- that's how the Republican Party got its start).
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The question really comes down to how long a three-party system would last, and I suspect that the answer is not very long.
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The Republican Party was started as, basically, the anti-slavery party with a modernization agenda focusing on industrialization and mechanization as both economically and morally superior to slavery, and to improved higher education.
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The Whigs and the American Party (nick-named "The Know Nothings) basically tore themselves apart on the issue of slavery, with the abolitionists in those parties becoming Republicans and the pro-slavery advocates becoming Democrats. (I know it sounds wierd, but that's how the current system started out in the 1850's.)
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American political parties are not ideological in the sense that you find in Europe. In many western European democracies, parties tend to have an ideological agenda to which they stick quite closely, and when the agenda becomes no longer relevant, they tend to disappear and their members set up new parties along the fault lines of whatever the new topic of controversy is.
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The Republican and Democratic parties in the United States have tended to be highly flexible (some would say lacking in conviction) in their ideologies. Honestly, if I had to pick an analogous rivalry, Coke vs. Pepsi, or McDonald's vs. Burger King, would be more fitting than comparing them to the Tories and Labour in the United Kingdom. They're not about ideology so much as they are about market share.
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There are quite a few scholars who say that this two-party balance is a product of each party attempting to achieve a coalition that attracts the support of the majority -- which is much more difficult in the United States than in countries with more homogenous populations.
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When one party jettisons a part of its coalition, the other party is highly tempted to start courting them. When the Democratic party leadership began to aggressively pursue a civil rights agenda, we saw the creation of the States Rights Democratic Party (the Dixiecrats) and George Wallace's American Independence Party.
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Both of these parties ended up being absorbed into the Republican coalition.
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This kind of thing gives both of our major parties an ideological squishiness that bothers a lot of people, but it does have the virtue that both parties are continually struggling to build coalitions that attract a majority of voters. In a country as large and diverse as ours, that's not necessarily a bad thing.
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But it's messy.

How about a viable SECOND party, as it stands it's really just two wings of the same bird.

sadly, no
The dumbing down of the US continues, no one can think independently
it's either way left or way right
what's it like in AU?