Lyrical Toy Box is a merchandise CD, released for Triangle Heart 3 on June 29, 2001. It contains a large amount of fan material, including a number of visual novel-like stories called "mini-scenarios". One of them, entitled "Magical Girl Lyrical Nanoha", cast Takamachi Nanoha (a minor character in the TH3 canon) as a magical girl and served as a prototype for the Magical Girl Lyrical Nanoha anime series. Nanoha herself calls the Music Video section as the Opening of the anime.

Source: Nanoha wikia

AniDB | MAL

Parent story: Triangle Heart: Sweet Songs Forever
Alternative setting: Mahou Shoujo Lyrical Nanoha