Yes--of course a 3-way relationship can work. There's great variety in humans and in their sensibilities, their views of life, their preferred source of emotional sustenance, and their ability to give of themselves. So, as QB and others have said, the right people, with the right level of communication, with the appropriately diverse emotional makeup, can certainly throuple.
No--I don't believe human relationships have evolved that much lately. For all the talk about internet borne anomie and rootlessness, the state of things today may well reflect an unchanged desire for human connection. Accompanying all the stories about people who live their lives on Zoom or Tik-Tok are multiple stories about people disconnecting from the wire to reconnect in the real world. We're not that far removed--evolutionally--from the simple creatures who first stood on two legs not so long ago. Modern encumbrances and cleverly programmed systems are no match for an amygdala that harkens back to Homo Habilis. The emotional learning and programming in your amygdala is not about to be outdone by even the smartest motherfuckers at Facebook. To the contrary, they make money betting on that desire for connection.
No--I don't believe all the "talk" about different types of relationships means much without the data to suggest that it is affecting the actual behavior of more than a right-of-decimal-point percentage of humans. We humans talk a good game, but those hearts are still of passion, jealousy and hate. I've had the misfortune of hearing 20- and 30-somethings talk to each other (fucking painful!) and they may talk openly about porn, be willing to live the life of a digital nomad tramping through Asia,, forego many of the material trappings of capitalist success, and explore their own personal spectrum of sexuality. But I also hear them talking about relationships--the kind where two people get together and form some sort of bond; maybe not marriage, but something that feels concrete and committed.
Yes--I think there is still a "most people". There's always a pathfinder group willing to try new things. We've had communes, Westernized ashrams, nudist colonies, swingers, love-ins, and about 300 different types of societal -isms in recent human history. But the arc of change in the human brain takes a lot longer than a few decades of experimentation. I hope we all keep open minds and hearts and continue to test and dream and explore, but let's not kid ourselves about any of these changing base human nature anytime soon.