Help me understand Triumvirate VS Geos (yes, in 2022).

Basically with Triumvirate vs Geos, all your toons need to be tanky to survive the initial barrage. Typically you want CA arrow on Nihilus and Traya. Sion probably doesn't need it. Your gear level shouldn't be too low compared to the Geos.

Even if you lose Nihilus, that doesn't necessarily mean a loss. That's because when you get a turn, the Geos will have very low health due to all the out of turn attacks. One AOE from Sion probably wipes out the entire team.