A somewhat familiar or affectionate term for the third-born son in a family. Often used in contrast with 長男 (eldest son) and 次男 (second son). The suffix 坊 adds a casual, boyish nuance.
See also: 次男坊
Our third son is still in elementary school.
With the birth of our third son, our family became five.
次男 is the formal term for 'second son', without the casual 坊 suffix.
三男 is the neutral, formal term for 'third son'. 三男坊 is more colloquial and affectionate.
Compound of 三 (san, 'three'), 男 (nan, 'male'), and 坊 (bō, 'boy' or 'son'), a suffix often used in familiar terms for sons.