Three boys in a tub is one of the emblems of St Nicholas of Chancery.
boys will be boys used in (often ironical) acceptance of behaviour traditionally seen as determined by gender (and youth). Girls will be girls is also found. Recorded (earliest in the variant ‘youth will be youthful’) from the early 17th century.
two boys are half a boy, and three boys are no boy at all the more boys there are to help, the less work they will do. Saying recorded from the mid 20th century.
See also boy.