After reading EDE's Mary Marvel in the Future post, it struck me that the group of two males and a female is quite widespread, but by no means universal, in storytelling. Something that just happens or some sort of golden mean in team balance?
A few examples that came to mind:
The Legion founders, the original LSV, Mary Marvel story with three kids from the future and the basic Marvel family
The DC trinity of Superman, Batman, Wonder Woman
Harry Potter with Harry, Hermione and Ron
Lockwood & Co. (a YA series I've been reading): Anthony Lockwood, George and Lucy
The Ministry of Time series: a core team (Julien, Alonso and Amelia) and the Ministry management (Salvador, Ermesto & Irene)
I couldn't think of a similar pattern for Marvel; they either had a large group (X-Men, Avengers) or the Fantastic Four
There was a British adventure series, from either the 60s or 70s, with a trio of special agents consisting of two men and one woman, but I can't remember the title or the names of the cast members.
Department S was what I was thinking of. Growing up in Australia we saw a lot of British shows on our TV.
That article you linked to on TVtropes was interesting EDE and they had a couple of other examples in the Comic Book section below, namely original Doom Patrol which fits, and the X-Men triangle of Cyclops, Jean Grey and Wolverine, but that grouping isn't as strong because of being part of the larger group and all of them having strong ties to other characters.
Another good example is from Star Wars with Luke, Han and Leia.
Another that gets mentioned in relation to TV Smallville is Clark, Pete and Lana, but that applies equally well to the original comics as well.
I'm curious about the switched trio, two girls and a guy. Not as common it seems, probably due to the long history of male dominance in most societies. One that jumped out at me (once it was mentioned) was Archie, Betty and Veronica. In the originakl WOnder Woman we have Diana, Steve and Etta. On TV there was the British sitcom Man About the House which had a guy sharing a flat with two young women at a time when that was very uncommon.
Never even thought of looking at tvtropes! What don't they cover?
They included Arthurian legend with Arthur, Lancelot and Guinivere, a love triangle within a bigger story rather than a team with a mission. I was thinking it might be this love triangle/sexual tension aspect which made the grouping popular for stories, although it's also present in some kids' stories. On a percentage basis, I doubt it's any more popular than other combinations, it just stands out because of the Legion connection. There's something about a group of three which works better than two or four+.
Two girls and a guy, the examples of Man about the House/Three's Company and Archie/Veronica/Betty are played more for comedy. I'm trying to think of some serious takes but none come to mind.
It will be interesting going forward to see if the trio group changes to reflect more awareness and acceptance of gender diversity.
I can't believe I didn't think sooner of this two girls and a guy thing:
Avengers: The Gathering, my favorite superhero story of all time!
For those not familiar with it, it's Crystal/Black Knight/Sersi as Betty/Archie/Veronica, but as grown-ups (in body if not emotional maturity) and with superpowers!
EDIT: Credit where it's due, I'm not the one who came up with the Marvel/Archie analogy, it was the very smart and very talented Jay Faerber, who loves that story as much as I do.