Necrozma fits the trio only because of its ability to fuse with Sogaleo/Lunala, as well as being released in Gen 7, as well as having a somewhat similar origin to Sogaleo/Lunala in their base evolution.
Seeing how Zacian, Zamazenta and Eternatus were released in the same generation but they can be categorized as the gen 8 legends (which in this case a trio). Zacian/Zamazenta are more of a duo. In fact, aside from being extremely powerful the legendary wold/dog Pokemon have almost nothing in common with Eternatus.
So Necrozma, Sogaleo, and Lunala are a trio (The light trio, or solar trio). Zacian/Zamazenta are a duo and Eternatus is kind of its own thing.