You could have an alien species who has three sexes, two who inseminate or whatever is needed. And the third as an incubator. The third needs both others, the male and female or whatever genders you want to determine them to be.
A few options can happen. With both other genders the third will always have a set of twins one of each gender. Or, they will have one to maybe three at a time and each will be able to grow into a random gender.
If you take the first approach and always have them in twins one as a male the other as a female and the third gender which is a mixture of both and an incubator would have to breed with another incubator to create the third type of mixed gender child, which would only be one at a time or it could be twins as well seeings they would be the main reproductive source of the three genders they should out number them alittle bit.
This could be done for a few reasons, but the best would probably be for defense, two are better then one and three better then two. The mixed gender could cook and clean, the male could hunt and gather, the female could build and craft. Any could take care of the kids but probably the mixed gender who was staying home and giving birth, and again, I would imagine they would be the most important to protect. And the other two genders could do all of the providing for the third gender and the kids.
Edit - For a little more detail, the incubator could pull in eggs from the female with the male appendage and accept insemination from the female. But again, you can have the genders be what ever you want, 2 males and one female or vice versa each with their own strengths and weaknesses or something else entirely, I'm sure theirs some speculation on the web as to what a third gender might be.
The only reason I suggest making a new one or having them both be the same gender probably male over female is that, it would make it less complicated to explain if there wasn't two genders who could give birth, but hey, maybe it might help put a twist on your story if two genders could do that, if it was modern day there could be social issues with it, maybe some believe one gender to be gods birth giver over the other or something else that discriminates them in some way.