Okay, looks like discussion needs to be steered. And I think I've got a good idea here:
As of right now, according to Bulbapedia, 39 Gen VII Pokémon have been revealed. Some full evolution lines (Yungoos, Grubbin), some legendaries (Magearna, Tapu Koko), a lot of unevolved Pokémon, and quite a few presumed two-stage evolution lines. But do you think any of the Pokémon we've seen so far are middle evolutions? Charjabug nonwithstanding, of course.
During the XY pre-release period, we only saw five middle stage Pokémon: The starters, Spewpa and Doublade. There are few enough new three-stage evolutions in Gen VI that I can walk you through every one:
Starter lines: First and middle stages revealed pre-release (missing final evolutions) - middle stages revealed one month before game release, for the record
Fletchling line: Fletchling and Talonflame revealed pre-release (missing Fletchinder)
Scatterbug line: All three revealed pre-release (together, if I'm not mistaken, or at least over a short time span)
Flabébé line: Only Flabébé revealed (missing Floette and Florges)
Honedge line: Honedge and Doublade revealed (missing Aegislash)
Goomy line: Not revealed at all before release
It's a safe bet that the Alola starters are three-stage. We know Grubbin evolves twice. Pikipek is also likely to do so. These, however, are quite formulaic evolution families. Kalos gave us three "non-formulaic" three-stage evolution lines, and arguably early in the pre-release period we got news of two of them: Flabébé in June, Honedge in July. For reference, the games themselves were announced in January, but the "dry spell" of XY news (during which only Sylveon was revealed following the announcement) was broken in May's CoroCoro.
To recap, for XY, we had seen members of all but one of the 8 three-stage evolution lines three months before the games were released. However, only Scatterbug's evolution line was truly confirmed to be a three-stage line. When we saw Honedge and Flabébé, we had no idea they would evolve twice. Doublade was announced rather late (late September) and thought to be Honedge's only evolution.
My late-night logic dictates that there should be a "Honedge" or two among the Gen VII Pokémon we've seen so far. A Pokémon which is a member of a three-stage family, but without announced evolutions - or with one revealed evolution, currently thought to be its only one. Do you have any idea which?