Honestly, given the possible pretext of the 2 other characters being servants for one god/high figure, I honestly think it'll be less of a triple-boss fight and more of a boss fight with 2 humanoid familiars, similar to Miko's spellcard using Futo and Tojiko.
In regard to the possibility of having 3 options for Stage 6, it would be a little tedious I feel, and though it'd be cool for sure, I don't know if it's too probable, considering Imperishable Night already exists, in which that feature was its unique strongpoint. That's just a hunch though, so feel free to prove me wrong~
Another thing is that the 2 'servant' characters on the right and left have similar headgear and similar poses judging by the silhouette, which definitely establishes a strong connection between them, and so I doubt those 2 would be separate bosses. It also wouldn't make much sense for them to be on the front cover if they weren't affiliated with the final boss -- even Seija wasn't on the cover for DDC, despite being the true culprit of the incident. Therefore, I believe that the 2 'servant's also share a connection to the central figure, and will probably be used in her fight as familiars, possibly like Futo and Tojiko, or possibly like Yoshika to Seiga.
Just my two cents, though. :b