You don't think that's been considered? And simulated? You don't call the third sex shit, because it can't exist. You can't evolve it. Asexual reproduction, quantitatively speaking, is king. Evolution maximizes fecundity, and asexual organisms are the most fecund. Though it is a topic of HIGH theoretical interest, there is actually no consensus on how sexual reproduction managed to evolve at all. That said, there are advantages to sexual reprodiction, and you can get it to evolve in a simulation if you mess around with the parameters enough. What you get from sex is the ability to mix genes. You get a functionally, practically infinite number of potential gene combinations from sex, which is why sex can produce billions of offspring no two of which are identical. That's on top of all the other processes that cause genetic change in asexual, clonal reproduction. What is REALLY HARD to successfully simulate, model, or explain with an equation is how this mixing of genes increases fitness enough to survive in competition with a comparable organism that reproduces asexually. The asexual wins, crushing the sexual organism in any competition. It reproduces faster and doesn't have to spend time and energy finding a mate (and possibly failing!) One theory I've read suggests that the key may have lain in the immune system. Clonal organisms are much more vulnerable to pathogens over the long term. So it is easier for asexuality to evolve from sexuality than the other way around, but the sexual lineages had more staying power over millions of years. Getting to some familiar territory, there is evidence supporting this in vertebrates. Parthenogenic female-only lineages that are pretty young have evolved from sexual ones (i.e. the whiptail lizard) but they don't have enough staying power so that, say, any of the major families of vertebrates is entirely parthenogenic. So yeah, having three sexes has no additional advantage (functionally infinite remember?) while exponentially multiplying the disadvantage (every organism has to find and copulate with two partners). Such a system would be unstable even if it were poofed into existence and given time would become asexual or bi-sexual. The only way I can think of something like it even making sense is if you had two "males" that fertilized an "egg" that lacks genetic material, but I'm not sure that could be stable either given that an organism that exclusively produced "eggs" would be at a huge disadvantage itself (yes, evolution applies to different sexes differently, and there would be no selective advantage for the egg-layer to evolve behavior conducive to copulation...actually if anything it would only copulate with its closest relatives by kin selection and you'd get inbreeding depression right away). Granted you can get some odd stuff going on with eusocial animals like ants, where not every individual is fertile, but if you go and work out the genetics and the game theory each individual actually is working to promote its genes being spread (kin selection).