The numeral 'three' used with the general counter for small objects (みっつ / みつ).
りんごを三つください。
Please give me three apples.
箱の中にボールが三つ入っている。
There are three balls in the box.
問題は全部で三つある。
There are three problems in total.
An expression for age meaning three years old; used especially when referring to young children's ages (can be used instead of 三歳).
甥は今月で三つになった。
My nephew turned three this month.
その子はまだ三つだから、幼稚園に通っていない。
That child is only three, so he doesn't go to kindergarten.