From Bulbapedia, the community-driven Pokémon encyclopedia.
The Beast Warrior Trio (Japanese: 三獣士 sanjūshi) are new and powerful Team Rocket members who appear in the the Pokémon Special manga as part of the Pokémon FireRed and LeafGreen storyline. They are Giovanni's new lieutenants and are quite powerful and ruthless.
Under Giovanni's command, they raid the Sevii Islands looking for Red, Green, and Blue. It seems this was done to obtain data on Deoxys. After Giovanni captured Deoxys, they headed back to Kanto. All three of them have black Pokédexes, which they use to determine the opponents Attack stat.
The members are:
At the end of the saga, thinking that Giovanni was going to pass on the leadership of Team Rocket to Silver, Chakra overthrows the other Beast Warrior Trio members, as well as Giovanni, killing Ōka and trying to kill Saki, Red, Giovanni, and the others aboard Team Rocket's gigantic helicopter while doing so.
Saki, however, survived, and attempted to recapture Deoxys. She failed, but managed to petrify Red, Green, Blue, Yellow, and Silver in the process.
Their Pokémon are: