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So. I've had a concept for a three-gendered alien species: Male, Female, and "Carrier" for lack of a better term. Male has penis, female has ovipositor, and carrier has uterus.
I feel like this is something that would have some evolutionary hurdles to even show up, but I don't know nearly enough biology to be able to take even a guess as to what.
Species isn't necessarily sentient, although it could be. Society for a sentient race would be different, but that's well beyond the scope of this question.