What's a plausible way for a three biological sexes setup to develop for a dominant intelligent species?

It's easy enough to figure out how a species might evolve a third biological gender in general (e.g. some sort of asexual drones/workers, ala insects).

But what I want to build is a system where the species evolved 3 "real" sexual genders, where all three take active part in sexual reproduction.

What would be a realistic evolutionary path for such a species (intelligent, but not necessarily humanoid) to develop?

I would accept plausible theoretical answers, but would prefer those based on actual evolutionary biology material.

Assume that the environment is somewhat Earth-like, but not necessarily exactly Earth; and no "unnatural" selection pressure occurs, such as alien experiments, magic, or panspermia introducing genes for 3 biological genders from another planet.

My own assumption is that this is NOT the only species with 3 biological genders on the planet, but that's from my understanding of biology and not some needs of the setting.

posted over 9 years ago

DVK‭

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