Note: A lot of these are being transplanted from Bizarre Sexual Dimorphism. If you notice any other similar examples, please point them out.
Sillage has an alien species that apparently requires two "females" and a "male" for procreation.
The Soft Ones in Isaac Asimov's The Gods Themselves have three sexes: Rational/left, Emotional/mid and Parental/right, which reproduce by fusing their bodies together and "melting".
Known Space: The Puppeteers, who are already fairly strange looking, claim to have three "sexes", one of which is non-sentient, technically a different species, and serves as a host for a the embryo created by the two others. In actuality, this is a subversion. They're merely prudish about admitting that they're a parasitoid species
The dominant race of the Empire of Azad in Iain M. Banks' The Culture/The Player Of Games has three sexes: One is male, the 'apex' has ovaries and "a kind of reversible vagina", and the female has a womb. The only non-sexual difference between the sexes is the eugenically bred-in lowered intelligence for non-apices. Sexism here sees females as breeders and domestics, males as workhorses and disposable soldiers.
"Venus and the Seven Sexes" (1949), a short-story by William Tenn, features a seven sexed species that passes gametes in a chain: sex 'D' receives from sex 'C' and transmits to sex 'E'. The sex of the offspring is determined by the sex of the parent which receives/completes the fully fertilized gamete. One sex is tasked with coordinating the family.
The Tralfamadorians from Kurt Vonnegut's Slaughterhouse-Five claim to have five sexes; also, because they can see through time as well as space, they claim humans have seven sexes. It doesn't really make sense from the human perspective when some of the "sexes" are "male homosexuals", "women over sixty-five" and "baby". However, it could be that the other 5 sexes we don't percieve contribute to reproduction in ways we aren't aware of/can't percieve in the dimensions our senses work on. Each human would thus have 7 individuals who contributed to the act of procreation even though we can only detect 2.
Bob Shaw's "Warren Peace" has the Squelchers, an alien race with no less then six different sexes, each one with its own unique appearance, and with a reproductive cycle where each sex fertilizes the others in turn. The forms look so different that, to the vast majority of the universe, the species only consists of the fourth sex, which resembles an orange haired saggy sasquatch (kind of like a blown up balloon that's developed a slow leak) with multiple eyes in a ring around its head (usually covered by its fur), oversized feet that let it wade on water, and two giant red nipple-like gamete sacs positioned one above the other on its torso. The fifth sex, the only other one mentioned, is described as being indistinguishible from a tree, except for the presence of a pair of two dual-pronged ovipositors (they look almost identical to staples) sprouting from its trunk.
Chanur Novels: The stsho have "bizarre sexual trimorphism"; their sexes are called "gtst", "gtste", and "gtsto". If emotionally disturbed, they will undergo "phasing" and change sex as well as personality.
In The True Meaning of Smekday by Adam Rex, the Boov aliens who attempt to enslave the human race have 7 sexes that translate in English to: girl, boy, boygirl, girlboy, boyboygirl, and boyboyboyboy.
In the Star Trek novels by Diane Duane, there is one race, the Sulamid that is described as a bundle of bright purple tentacles about six to seven feet high, topped off with a sheaf of pink-stalked and tentacled eyes with triangular pupils and a purplish, "bloodshot" look. According to Dr. McCoy, they have twelve sexes, and all of them claim to be male, especially the ones that bear the children. The Enterprise has at least two of them among the crew, Mr. Athende in Maintenance and Lt. Meshav, from Data Management.
In Octavia Butler's Xenogenesis books, there are male and female Oankali, but the third gender Ooloi does the genetic mixing.
In Pier Xanthony's Cluster series, the Spicans have three sexes - impact, undulant, and sibilant - of which all three are required for reproduction.
In one of the Star Trek: Enterprise episodes there was a race that had Male, Female and a third intermediate sex ("Cogenitor"). Trip develops a friendship with the third person in a trio of aliens and had a difficult time when he finds out that the cogenitor sex is treated as an inferior.
Alien Nation has Male + Female + Catalyst. Catalysts are very rare, but necessary somehow. Once mating has happened, the egg spends sometime in the mother's egg pouch and sometimes in the father's egg pouch.
The Droyne in Traveller have three sexes: Male, female, and enabler, the enablers being required to give off scent during a mating session. Droyne also are casted in the manner of social insects and each caste has only one sex. Droyne language focuses more on caste then on sex as that is more important in their psychological framework.
The Grekim from the RTS Achron have three genders (octo, pharo, and sepi). They also have three 'classes' (basic, pod class, and ligo class). Any two different genders can mate to produce a member of the third gender of the same class or the next one up. E.g. a pharo and a sepi can produce an octo or an octopod, or an octopod and a pharopod can produce a sepipod or a sepiligo. This is actually the species' primary method of producing units in the game. Ligo units cannot mate (or 'progenerate'), but can 'split down' into two basic units of the remaining genders. E.g. an octoligo can split down into a pharo and a sepi.
Cthulhu Saves the World has party member Paws, a cat-like alien, explain that his species technically has every member as a unique gender. However, for sake of convenience, he allows the party to refer to him as "he".
Done with two different species in the X-Universe. The Boron have three sexes: male, female, and Lar. The presence of a Lar during reproduction is highly valued, though not strictly necessary. Meanwhile, the Paranids have eleven genders, and while not all of them are required for any individual act of reproduction, the combination of parents will affect the development of the young.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.