noun
three-year-old
Refers to a child of three years of age. Often used in contexts like 三つ子の魂百まで (the soul of a three-year-old lasts a hundred years).
三つ子の魂百までというから、幼い頃のしつけが大切だ。
As the saying goes, 'the soul of a three-year-old lasts a hundred years,' so early discipline is important.
noun
triplets
Refers to three children born from the same pregnancy. The word is the same as the 'three-year-old' sense, so context is essential.
彼女は三つ子を出産した。
She gave birth to triplets.
From 三つ (three) + 子 (child). The reading みつご is a regular combination of みつ and こ, with rendaku voicing of こ to ご.