Charlie is a Greek god and his genetic material magically combines with Alice and Bob's.
Charlie's isn't literally an extra parent or even male for that matter: They, Alice and Bob are aliens whose species naturally mates in trios. This doesn't stop Charlie from helping the verymuchhuman Ted and Carol conceive Alf.
A multiple birth (twins, triplets, quadruplets, etc.) are born through normal sexual or asexual reproduction.
Subverted: David has multiple parents and seems to inherit a lot of their traits, but only two are, in fact, biological.
Double Subverted: ...And then it's revealed that the other parent(s)' traits don't pass down genetically, but through life force or memory or something of that sort. David was partially conceived by them without a doubt, but not in a method most humans would understand.
Parodied: Charlie is an alien whose species can conceive with an arbitrary number of parents of any combination of different sexes and species no matter how illogical that is. He successfully conceives the healthy twins Elaine and Gary with the help of their happy parents Alice, Bob, Carol, Ted, David, Alf, the citizens and livestock of a small town, a warehouse full of demonically-possessed 1958 Plymouth Furies, a backyard tomato garden, a cemetery, and thousands of miniature giant space hamsters.
Zig Zagged: Alice, Bob, and Charlie all confirm and deny being David's parent at a given time, and genetic testing is inconclusive.
Averted: Human DNA turns out to be incompatible to Charlie's race, and conception is impossible.
Enforced: There's been a Love Triangle going on between Alice, Bob and Charlie for some time, so the execs decide to solve it by having the three of them conceive David so they decide that all three have a relationship.
Lampshaded: "Charlie's species just happened to have the right genetics to allow all of us conceive a child! Isn't that amazing?!"
It's a Free-Love Future, and two-person couples are now ancient history. In order to support this new dynamic, scientific breakthroughs have created a third, fourth, or even fifth sex to produce offspring.
Alice and Bob are aliens who can conceive just fine with only two parents of different sexes, but would prefer to include a third parent of another sex like Charlie.
Exploited: Both Ted and Carol want Charlie, but know that Charlie's species can reproduce with any number of mates, so they decide to join forces in seduction.
Defied: Despite living in a Free-Love Future, Alice and Bob are soulmates and are sure that their baby will be just fine with two parents.
Right after "the bussiness" has been done, this happens...
Charlie: You know, my species can mate with any number of partners. Carol & Ted: Reeeeeally?!
Alice, Bob and Charlie are a menage a trois, and wish that there were a way to conceive a child with all their traits.
Deconstructed: Alf inherited not just all of the good traits that come with each parent, but the bad as well. He's born with the Type 1 diabetes inherited from Carol, weak bones from Ted's side of the family, and Charlie's species is sometimes has a rare disease that makes plant chlorophyll allergic.
Reconstructed: ...But despite all of that, Alf eventually gets past all of his deficiencies to live a wonderful life. Also, his unique genetic heritage has helped open the door for many medical discoveries.
Played For Laughs: David has just arrived in Orgyburg, and everybody he meets treats him as a son, including the animals.