The man in the middle is a father and a son at once, so he counts twice.
Strictly the grandfather is somebody’s son too, which makes two fathers and three sons, but nobody likes that answer.
Two fathers and two sons go fishing. Each of them catches one fish, and they bring home three fish between them. None were lost or eaten. How?
Give me a hintThe hint
Count the people, not the titles.
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There are only three of them: grandfather, father and son
Source: Traditional