PLoS Biology: Everything You Always Wanted to Know about Sexes: Open-access article, discussing this argumentation:
"No known organism needs more than one mother and one father. But even this assumption is now starting to break down at the level of biological systems. In a recently discovered hybrid system within the harvester ant genus Pogonomyrmex, queens must mate with two types of males to produce both reproductive individuals and workers. These ants are the first species known which truly has more than two sexes with colonies effectively having three parents argues Joel Parker of the University of Lausanne, Switzerland."