^ I actually have some theories on that, if you want to read them. (Granted, I'll be writing them down either way...)
A trio HELLO!! probably won't happen until they do whatever it is they're planning to do with 876pro, most likely a game. They'll add the trio version to the PS3 game, though the Portable game for 876 (and I'm pretty sure 876 will stay on a portable system. Mostly because there hasn't been a true portable imas game since DS...) will probably stay with solo-only.
Another idea, they won't release HELLO!! for a while; they'll release variations on the current 876 song packs. For example, we might get 876 Pro★Cool Ai, which would be her Cool costume (in pink) and unlock Purikogu and Shiny Smile for her, while 876 Pro★Cosmic Ai would be her (pink) Cosmic costume and unlock her Dazzling World and Kiramekirari. Rinse and repeat until complete. Would be a complete money trap... that I'm sure would be successful. (As a bonus, though, unlocking the songs on multiple idols would allow duo or trio performances of that song with those idols.)
Or maybe they'll do an anime thing with the 876 girls before a game thing, since there was that vaguely cliffhanger-ish hint at the end of the anime. Then the HELLO!! thing would coincide with that.
... as a final, worst-case-scenario thing, they might not release HELLO!! for them.