Due to no legendary trio and lack of post game and ****, I wouldn't at all be surprised if this was real. But I'm takin' it with a grain of salt because I don't really care idk????///
Anyway, it's legit, I just searched for that Goomy on the GTS and I found it, asking for a Volcanion. I'll also try find the other two, as well.
Also too lazy to get my phone, so I just took pics with webcam, sorry that they're LQ and backwards
Haven't been able to find Hoopa because haven't seen an Anorith. Okay, found it by searching Xerneas, the other two pokemon were also listed in Xerneas, so I guess just searching Xerneas would be easiest way to see all three names in one search (I'd assume the same would work for Yeveltal, too). Btw if you want to see for yourself, go on the GTS and search the following;
To see Volcanion: Search Goomy, female, level 31-40
To see Diancie: Search Flabebe, female, level 11-30 (there's also a Volcanion in here, too)
To see Hoopa: Search Anorith, male, level 11-20