Magic Knight Rayearth a manga series (1993-1995) created by CLAMP, with a total of 6 volumes. The anime (1994) has a total of 49 episodes split into 2 seasons. The first has 20 episodes and the second has 29 episodes. There's also an OAV series (1997) which has three parts but it has nothing to do with the original story. There are also video games based on the series.

Shiidou Hikaru, Ryuuzaki Umi & Hououji Fuu were all at Tokyo Tower for an outdoor field trip. At the viewing tower, they heard a girl calling for help, and after that were pulled into a completely different world, Cephiro, a land of will. Waiting for them was a mage called Clef. He told them that Cephiro was facing great danger. The three of them were Magic Knights and were being called over by Princess Emeraude, the Pillar of Cephiro. If they did not defeat Zagato, the priest who had captured Princess Emeraude, and save her, they would not be able to return home.

So the three of them began their journey. But it wasn't smooth-sailing. They met different people - including those who supported Princess Emeruade with all their heart and those who were sent by Zagato to finish them off. At the same time the three girls build a strong, lasting friendship. In the end they successfully awakened the the legendary rune gods and defeated Zagato. But at the end of their journey to save Cephiro, something unexpected awaited them. *

The three girls were back at Tokyo, but when they met up at Tokyo Tower, they were called back to Cephiro again by someone unknown. The current Cephiro had now lost its Pillar and was facing destruction. At the same time, outside countries - Autozam, Chizeta & Fahren - were attacking Cephiro in hope of becoming the new Pillar of Cephiro. Upon hearing this the three girls decided to help fight for Cephiro. As the situation became more and more heated, Mokona - the creator of Cephiro - decided to chose the one with the strongest heart to become the Pillar. Hikaru and Eagle, of Autozam, were the chosen candidates. But in the end only one can become the Pillar of Cephiro. What will Cephiro's fate be!? *

*Translated and edited from "Clamp no Kiseki, Volume 4", hence the plot is based on the manga.

In the anime, the first season is pretty similiar to the manga, except for the fact that Presea dies in the process of helping the Magic Knights. This sets the path for the second season with the addition of new characters - Debonair (you may have seen her adaption in the likes of Tsubasa Reservoir Chronicle), Nova and Sierra (twin sister of Presea).

In the OAV, the setting is Tokyo, not Cephiro. The three girls are good friends who go to the same school, but will soon separate ways for high school. Mokona, supposedly a fairy, appears before them (first spotted by Hikaru), and at the same time Tokyo is invaded by strangers alongside monsters. Clef then appears, using most of his power to freeze Earth and its people so as to be unaffected by the battles to take place (except for the three girls). He finds them and guides them, to awaken their Mashin and fight against the enemies invading Earth, who happen to be led by Eagle.