Case Closed: Conan and Heiji and the Vanished Boy(2003)

Meitantei Conan: Conan to Heiji to Kieta Shounen

Conan and Ran are visiting Heiji and Kazuha in Osaka but while they are touring around they come across a group of three kids that look like Ayumi, Mitsuhiko, and Genta. The group are looking for their missing friend and Conan agrees to help.

(Source: Anime News Network)