Imagine a world with intelligent beings [intelligence equal to human beings]. In general things are like here on Earth, except there are 3 partners required for reproduction [e.g. one sex with the usual stick-like appendage, one sex with the usual receptacle/cavity, and then a third sex with e.g. a hoop appendage that must be between the stick-like appendage and the receptacle].
Say that all the usual reasons for having a child are still valid: parents want a kid to experience youth/growth/learning, want to mold a young mind in their image, want a financial heir, want someone to take care of them when old; or accidental/unplanned birth; or rape; or uniting family lines for legal/financial/power reasons; etc.
My question is how does this 3 parents for reproduction arrangement affect the legal rules and rituals of child custody? (For sex A, B, C, there can be 4 divorce states: 3 ways to have 2 "remain married" while one separates, and 1 way for all 3 to separate)
Also, how will the legality change when there are N such partners, with N>3?
*Also, I know someone might say that humans now are in some sense a "3 or more parent species" for some of the following reasons: 1) surrogate mothers 2) the mitochondrial genome is separate and traces the maternal lineage etc. 3) genetic engineering introduces genetic changes that potentially neither biological parent has...
But the 3 or N parent species I'm talking about has equal contributions of genetic material/responsibility, etc.