You didn’t specify if you meant only Galar Trios, so I am going to list all the Trios/Duos/Quadruples(?) that you can find in Galar, except ones from the Max Raid Dens. (Except Necrozma)
Hero Duo
| 
Zacian & Zamazenta
Location: Galar | Post-Game Story
Light Trio
|
| 
Solgaleo, Lunala, & Necrozma
Location: Crown Tundra | Old Lady’s House as Cosmog (Solgaleo/Lunala) Max Raid Den (Necrozma)
Swords of Justice
|
|
| 
Cobalion, Terrakion, Virizion, & Keldeo
Location: Crown Tundra | Side Quest Sonia Asks of You
Galarian Legendary Birds
|
| 
Articuno, Moltres, & Zapdos
Location: Galar's Wild Area (Zapdos) Isle of Armor (Moltres) Crown Tundra (Articuno) | After you trigger them.
Notes:
Unlike SSuperpriority, I do not consider Gastier and Spectrier as a Duo, because in the the timeline that one exists, the other one does not. I also do not consider Calyrex as a duo with one of the horses, because they are two vaguely different Pokémon.
Eternatus is not possibly part of the Hero Trio, because it is on no way a Hero. Also, like Calyrex, is a very different Pokémon to the Hero Duo.
The two forms of Urshifu are not a Duo, because like SSuperpriority said, they are the same Pokémon.
Hope I helped!