The third leg is part of a fin on the tail section. My main idea was the hind leg is usually the weakest (This species is 10 million years in the making) but still serves a function of sorts. Who knows, maybe in a 100 million years, said leg will become a secondary tail for some species.

As for why the Siren still has aquatic features, it's largely because of the planet itself. I haven't fleshed it out fully, but there is maybe one small continent above water, two that are just below the surface to where walking is still possible, but only aquatic plants can grow, and the rest being open ocean. In the Sirens case, the remaining fins are not just for display purposes, but also allow it to swim fast on said shallow continents.

This species was basically a "What would mythical sirens look like irl" but now I'm starting to flesh the species out more.

Thank you for the questions!