It is a third (or fourth) phenotype, not a third sex. The abstract of the source article is careful to spell this out; the clickbait site introduced the error.

It is a third (or fourth) phenotype, not a third sex. The abstract of the source article is careful to spell this out; the clickbait site introduced the error. "Three sex phenotypes in a haploid algal species give insights into the evolutionary transition to a self‐compatible mating system." -- i.e. how might self-pollinating plants have developed?