noun, noun which may take the genitive case particle 'no'
Ordinal number 'third'. Used as a noun or with の to modify another noun (e.g., 第三の選択).
彼は第三の候補者として選ばれた。
He was chosen as the third candidate.
第三の問題は最も難しい。
The third problem is the most difficult.
三番目 is a more colloquial way to say 'third' in a sequence, while 第三 is more formal and often used in titles, categories, or abstract ordering.
Compound of prefix 第 (ordinal marker) and 三 (three).