This is a general counter for three unspecified items or "three pieces." It is commonly used for counting objects that do not have their own specific counter suffix, such as "three candies" or "three ideas."

形(かたち)や種類(しゅるい)を問(と)わず、物(もの)を「三(みっ)つ」数(かぞ)えるときに使(つか)います。お菓子(かし)三(みっ)つ、アイデア三(みっ)つなど。