Waiters. A Fire-type user, a Water-type user and their head waiter, Dento, A Grass-type user. Most likely each having only a single Pokémon.
In Ruby and Sapphire Tate and Liz were introduced as the first (and to date last) double battle Gym Leaders. So having more than one leader at once won’t be a new thing.
What this means has yet to be revealed but could be that Dento greets you in the beginning then you proceed to battle each leader with Dento being last. Or it could even be a triple battle~? Only time will tell!
P.S-Don’t forget to check out the vids in PJ’s last post. They’re awesome!
P.S.S-Ozy found some pics. He believes them to be fake.