(from DMP): Three extraterrestrials are on a mission to see if Earth is qualified to be part of the Galactic Control which is an order where only certain planets can be part of. If Earth isn't qualified then the planet will be destroyed. To progress in their mission, the three beings abduct three earth animals as disguises for themselves. With their extraordinary powers they already have, they were best known as the Wonder 3.

Sentai can be translated to "task force" and a lot of Japanese anime/manga/live action shows are generally aimed at children with the main characters as protectors of Earth where they can transform and gain superpowers. Ever seen Power Rangers? That's a Super Sentai series.

Wonder 3 takes place in an alternate Earth in the year 196X and the world is overrun with war and violence. The Galactic Federation consisting of all the advanced species of the universe convene to decide the future of Earth, but due to a lack of information they decide to send an elite team, Wonder 3, to observe human species for one year and determine whether or not humans are worthy of saving. Upon landing on Earth, the three members of Wonder 3 - Bokko, Nokko, and Pukko - disguise themselves as animals and come across the two Hoshi brothers who defend humanity in their own way. With a time limit of one year, what can Wonder 3 learn from the Hoshi brothers about life on Earth?

Official English

Volumes 139-141 of the Osamu Tezuka Complete Works