But we already know that Byleth is the main player character, which means if they're gonna put a Three House character in it's going to be Byleth. Edelgard is basically one of the siblings from Fate, so it's unlikely she's going get in. The only reason why people consider her over Byleth, Dimitri, and Claude is because of how little was know about Three Houses during it's first showing and she was the one character that people noticed. If there is a Three House character (which there shouldn't since that game comes out in 2019 and Rex & ARMS was deemed 'too late' to get into Smash as DLC) it's going to be Byleth.
First and foremost. If Three Houses are getting a DLC and not Rex and Arms, it's because of marketing material and nothing else, just like how it would be if Arms and/or Rex would be DLC if their games came out this year or beginning of next and FE3H did not get one and got released in 2017. Let's not kid us self thinking differently
But you are absolutely right that going by the patterns, they would most likely do Byleth. HOWEVER.... while it is said that Corrin got in because of marketing, one of the key things of why specifically Corrin got in was because of potential for original moveset. What I'm trying to say that if Nintendo truly wants a FE3H rep in for marketing, and Sakurai would be more comfortable adding her (or Claude/Dimitri for that matter) for the sake of diversity in moveset, then they would be stupid to force his hand with only Byleth
Heck, who knows. Edelgard is the most prominent character of the three students in all of the advertisement. Maybe they are using her like that just because they already decided to use her as DLC xD (While I'm not completely serious, it wouldn't shock me if that's the cause either)
Someone in the Geno thread came up with the idea of Byleth pulling a Pokemon trainer and comanding the three students. I doubt it would happen, but it is still kinda cool to think about.
I would not worry too much about Byleth. Sakurai would prioritize moveset uniqueness for sure and it helps that the three house leaders are super prominent as well. I also like to think that FE devs decided to take mercy upon Sakurai and give him 3 prominent and important FE character options that do not rely solely on swords.
First off, "insert Pokemon Trainer joke here" although that sounds kind of neat, speaking as a FE fan and Pokemon Trainer fan. But yeah, I agree with you. If we really are getting yet another FE rep, then Nintendo is idiots if they would force him to only focus on one character.
If Fire emblem ever gets another pure sword user, then the only character that would arguably go over well by general smash fans, its this guy due to design and all that jazz.