A three-year-old child (archaic or old-fashioned term; sometimes written 三つ児).
庭で三つ子が元気に遊んでいるのを見かけた。
I saw a three-year-old playing energetically in the garden.
昔話には三つ子の子どもが出てくることがある。
In old tales, a three-year-old child sometimes appears.
Triplets — three siblings born at the same time (three children from the same birth).
彼女は昨日、健康な三つ子を出産した。
She gave birth to healthy triplets yesterday.
三つ子はそれぞれそっくりで、見分けがつかなかった。
The triplets looked so similar that I couldn't tell them apart.