Strange duck behavior? Female with two males?

We have an inground pool and every year we get ducks. Last year we started getting a trio; two males and one female. One male is a typical mallard the other is a black duck. Well I am confused as to why this female has two males? And after a while (I'm assuming the female is nesting and that's why she isn't here) the two males come together by themselves. I find this to be very strange and have never been able to get a very clear answer as to why they do this. Does anyone know?