What is the final Aniimo evolution stage called?

The practical reason to care about the third stage is Ultimate moves. They are exclusive to final-form Nova creatures, so a species sitting at Lumin or Gamma does not have one and cannot get one by any other route. Evolution also raises stats and adds new abilities for most species, but the Ultimate is the part you cannot replicate through other progression.

Getting there is not automatic. Evolving a captured Aniimo costs specific resources, and certain evolutions want a condition met or a progression milestone reached rather than just more levels. Not every line has a Nova waiting at the end either. Some species run the full three stages, some stop at two, and some do not evolve at all, though the majority can. Flutternym evolves into Gracewing, and the name on its own tells you nothing about which stage that lands on.

A species that tops out early is not dead weight. Aniimo get stronger through standard leveling, traits, potential spent as Capability Points, resonance progression, carried equipment and runes, and none of that is gated behind reaching the final form.

Catching a Nova instead of evolving one

Alpha bosses hand you an already-evolved Aniimo. Catch one and you skip the resource cost and the conditions entirely, which is the fastest way to have a final-form creature on your team early. It only works on what the Alpha actually is, so it is not a substitute for raising a specific species you want.

Starter transformations are not a Nova form

Your starter's transformation is a temporary combat power boost, not a permanent evolution. The creature does not advance a stage and it reverts. If you are looking at your starter mid-transformation and wondering whether that is the final form, it is not.

Not settled: Which lines actually reach a third stage, and what each one demands to get there, differs by species, so a creature having a Lumin and a Gamma form is no guarantee a Nova exists for it.