Is there a team preview of 6 Pokémon which you see before picking, like in VGC? Anyway these are my thoughts on the most popular Pokémon in the tier and how they would fare (usage according to the latest stats of course).
Kyogre- The usurper king of Ubers does well in this metagame. Swift swimmers are quite useful as stall isn't that prevalent, which makes the speedy fossils and Kingdra quite the cleaners, even moreso than they are in the real metagam. Also Kyogre's power is great, and since it doesn't have to set up, its hard hitting STAB-moves are great in a metagame where you want to deal as much damage as possible quickly. A downside of Kyogre is IMO that with three Pokémon on the team and a STAB that depends on rain being up, as strange as it sounds, makes Kyogre set-up bait for a lot of stuff should the opponent have a Groudon. And considering switches aren't as good in a fast paced 3v3 game, this might be very detrimental. In all Kyogre is worth using. Make sure to get around set-up sweepers on sun teams (E-killer, Ghostceus etc).
A good set here would be Thunder Wave lure, with Lum Berry for random status or a Water Gem. Since matches are fairly short, Leftovers is probably not that great, and Focus Sash is also a bit iffy as Kyogre can usually take one move from most things. Roar is a cool move if you don't want to be set-up on in the sun. Scarf or specs are also quite cool, but beware of the drawbacks of using choice items in a format where switches can be really costly...
Arceus- E-killer gets into this new meta with ambition and high hopes. Surely, it will thrive vs many offensive teams as its bulk makes it very easy to set-up and sweep frail teams. The downside is that Arceus usually wants some chip damage on stuff before it can actually OHKO them, even with Silk Scarf. The chip damage might come from Stealth Rocks, and they are sure viable considering all the sashed mons. Really, Extremekiller will be great in this meta as well, I don't see how a Pokémon with a strong priority, great bulk, ability to sweep offensive teams, and ridiculous ease to set-up wouldn't be one of the best Pokémon to use here.
Mewtwo- Mewtwo faces much competition from Latios in the current meta imo. I always prefer Latios as wallbreaker as it is more hazard resilient and has much more ease to come in considering its resistances. I know many people disagree with this comparison, but I am just putting it out here. Now with Soul Dew gone and a metagame where once again, switches are less prevalent, Mewtwo finds a great niche. Being fast, armed with coverage for days and an awesome special attack, Mewtwo can dish out serious damage. One thing to note is that Mewtwo isn't that frail, it can usually take a hit from some stuff.
Darkrai- Obsolete here, sorry. The main niche Dark Void is gone :(
Groudon- Amazing tbh. Flipping the tables and taking away the rain, enabling a set-up sweepers to get that free turn to sweep is cool in this format. It can also set rocks, although, I'd probably use a nice double Dance or RP set. The strong Earthquake is capable of 2HKOing so much stuff. Just have a method to break sashes so it can cleanly sweep though. Choice Band is also an interesting option, as well as T-wave+Swords Dance which provides team support and sweeping capability in one set.
Palkia- As always, Palkia is a big threat and a great check to Kyogre too! Its awesome coverage is the main selling point here. Not much else to be said really, Palkia is amazing anyway. Just use it.
Genesect- I don't know it can be as good as it is in Ubers, U-turning is not that great here but a special scarf-set can still do good cleaning.
Dialga- Will probably be another of those really great mons here. The versatility it has as well as typing just owns. Scarf, specs, LO, SR are a couple of cool sets it can run efficiently here.
Rayquaza- Rayquaza is usually cool vs stall, but has trouble against fast offense. A sash set is viable here, with Extremespeed serving as a failsafe to pick off opposing Focus Sashes. Good attacking stats, good coverage and priority makes Rayquaza a solid option. Also, checks weather sweepers which is great!
Lugia- Opposed to contrary belief, I still think Lugia is a cool mon to use, but it is of course much worse in BW2 Ubers. In this 3v3 meta, filled with offense, a stall mon like Lugia might seem out of place. I think the opposite stands true here. Lugia will be epic here. SR being used, but less prevalent makes Lugia's multiscale something you can rely on even more often. Against offensive teams, Lugia has an ace in the hole: Thunder Wave which just owns so much. Paralyzing fast threats so slow hard hitters like Kyogre, Groudon, Kyurem-W etc can outspeed stuff is just amazing. And running Toxic on the same set is not only viable, but recommended in my opinion. Lugia will be cool here.
Giratina-O- This thing has many options, ranging from the usual stuff, mixed sets, T-wave support and Tailwind! It also has priority which is important to pick off Focus Sashes.
Ho-oh- I rather fancy Ho-oh in this meta as well. Well, it will probably not as good here as in regular Ubers as it can abuse Regenerator much more, at least if you prevent SR from getting up. Tailwind is really nice here, considering how battles finish faster than 6v6. Ho-oh also has Brave Bird+LO recoil to take itself out and let something else abuse the remainder of the turns.
Ferrothorn- It is actually OK I think. Stealth Rocks are still good, and Iron Barbs can break Focus Sashes on stuff with contact moves. Since swift swimmers have a good niche, Ferrothorn gets a niche by checking them. It also has Thunder Wave, which is just great.
Well, there is much more to discuss obviously, just wanted to put my stuff out here. Also, I am giving thumbs up for Custap Berry Skarmory. It can Taunt, phaze set-up sweepers and get up hazards of its own. I think it will be really great in such a metagame.