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

yes, you could almost always expect this type of RNG to happen. some stuff you can do to help is, speed up nihlius to be second, that way he can decrease cooldowns, and try to make sion weaker than the rest, Geos typically go for the weakest enemy first which sounds like that's Nihlius for you currently.
and as others have said Nihlius isn't a key to win either its mainly traya's lead that kills off the Geos