the three great ...; the big three ...
Prefix used to refer to the three most prominent or representative things in a category, often in fixed compounds like 三大疾病 (the three major diseases) or 三大都市 (the three major cities).
See also: 三大疾病
The three great festivals of Japan are Kyoto's Gion Festival, Osaka's Tenjin Festival, and Tokyo's Kanda Festival.
Speaking of the world's three great delicacies, they are caviar, truffles, and foie gras.
the Big Three (countries)
the three grand national holidays (pre-1927)
the three great desires (sleep, food, sex)
three major diseases
the three major cities
three greatest teas of Japan
Compound of 三 (three) and 大 (big, great). The prefix usage is a straightforward Sino-Japanese formation.