Ho-oh: The reason I brought up ho-oh is its typing and speed tier- basically everything everything that checks and counters charizard, also counters this thing, and although it has a lot more bulk, it is slower in regular play opening it up to new threats and still under sped by most mons in trick room. Although yes there are very little stealth rocks in this meta, rock moves are everywhere between Mega Diancie and the glut of fast rock slide abusers- Landorus, Garchomp, Terrakion, Aerodactyl among others. There is also the threat against rain and strong water type spam, as well as mons such as Mega Manectric, which although unable to ohko, can deal some serious damage for an opponent. I think it would be much easter to handle without putting too much stress on team building and would add a really interesting offensive or defensive option for your team.Ho-oh effectively loses one of its biggest disadvantages (Almost nobody uses Stealth Rock in Triples), and meanwhile it's monstrously bulky and plenty hard hitting with, of course, Sacred Fire access. I'm not seeing it unbanned anytime soon.
Mewtwo is absurdly flexible, lightning fast, and very powerful. While I'd love to run it so I could have another Hurricane user in Rain (And Gravity), it's completely bonkers to unban it in Smogon Triples.
Groudon is also plenty powerful.
Regular Rayquaza is... not overly powerful? Maybe? It's Speed tier is painful...
.... honestly, looking over the list of bans (It's small), regular Rayquaza is the only one I'm spacing on a good argument against unbanning it, which isn't the same thing as saying it's a good idea. It still offers a crazy powerful Draco Meteor, it has access to V-Create, it's the superior weather-block choice by far, and 95 Speed is still outrunning a lot of things -and there's tons of things you can do to support it, like setting Tailwind.
Mewtwo: yeah in hindsight this thing is too powerful, it's speed tier means that it can nuke most of its checks and counters and with a little quick guard support this thing isn't dying without taking one or two mons doing it, all with its superb supporting move pool.
Groudon: On the fence with this one. Adds another check to rain teams and gives sun incredible support with or without Charizard Y. It's not its attack power but rather it's bulk that in hindsight I worry may be too much for the tier. Although I'd like to see it tested, I won't get my hopes up.
Rayquaza- As much a Nuke Dragon ascent ray is, it's slow speed tier, typing and frailty after using the move leave it a sitting duck on the field. Extreme speed and V Create give it unique coverage and priority, but I think in the three on three meta it wouldn't be overpowered. Rays probably a lot more splashable on teams than ho-oh, but similarly, it's speed issues mean it faces threats from pretty much every dragon check and counter in existence. Fairy's, ice and dragon types are in no shortage in triples and I think he'd make a top tier threat again without being broken.