Rosuto Ninpū Tankyūger

Rosuto Ninpū Tankyūger, or "Lost Ninja Quest Ranger," is the Sentai series off which the newly-begun Power Rangers: Lost Ninjas is based.

Synopsis
So far, all that has been revealed is that the three Terra Rangers are brothers, sons of the Sensei. He helped his youngest and favorite, TerraBlue, to escape, and his brothers willingly joined the villain out of a mix of betrayal and revenge.