Quarrelsome Boys

A father took his sons out for a picnic and had to cross a river in a small boat. The boys were born one year apart. Two boys would fight each other if their ages differed by one year, provided that the father was not around to stop it.

Given that only the father could row, how could this family cross the river peacefully, if

there were three boys and the boat held two people;
there were five boys and the boat held three people, and the father could stop fights on the boat;
there were five boys and the boat held three people, but the father could not stop fights on the boat?
(From T. H. O'Beirne's Puzzles and Paradoxes.)