Son, Dad, Grandpa.
one is the GRANDfather of another one's son.
father - son - step father
Ah, but there is no blood relation between all 3, if one is a "step" father.
Hmm, typically the riddle doesn't specify that they're men and the answer is mother/father/son.
Perhaps the answer is as simple as "father & son"
A man has a wife, and they have a son.
The son grows up, and has sex with the mum, producing another child. The man is now the father of the older boy, and is still blood related to his wife's new baby. He thinks that baby is his son, even though it's really his son's son. His weird older son had sex with his pervy mum to produce that younger son.
Now where's my prize?
that's just not right.
i think the real answer is they are not all men, and one is the mom, like someone already stated