noun
three-piece suit
Refers to a matching set of jacket, trousers, and vest (waistcoat).
彼は結婚式に三つ揃いのスーツを着て行った。
He wore a three-piece suit to the wedding.
スーツ is a general suit, while 三つ揃い specifically means a three-piece suit with a vest.
Compound of 三つ (three) and 揃い (set, matching). The reading みつぞろい reflects rendaku on そろい.