Sonic Underground is an animated series co-produced by TF1, DiC Entertainment (which was owned by Disney at the time) and Les Studios Tex. It is the third Sonic the Hedgehog animated series, and also the last one produced by DiC; it follows a main plot separate from all other Sonic the Hedgehog media, where Sonic had two siblings, Sonia and Manic, that were collectively part of a royal family who were forced to separate from their mother, Queen Aleena, upon Robotnik's takeover of Mobius due to a prophecy told by the Oracle of Delphius. It first premiered in France on 6 January 1999 on TF1, and then premiered in the United Kingdom on 2 May 1999 on ITV and finally in the United States in syndication on 30 August 1999 and ended on 22 October 1999.

The show takes place in a separate canon and continuity than any other Sonic the Hedgehog media. Queen Aleena, the former ruler of Mobius, was overthrown by Dr. Robotnik and his lackeys Sleet and Dingo. Robotnik seized control of the planet and forced Queen Aleena into hiding. To preserve the dynasty, Queen Aleena separated her three children: Sonic, Manic, and Sonia after the Oracle of Delphius told her of a prophecy, proclaiming that one day, Queen Aleena would reunite with her children to form the "Council of Four," and overthrow Robotnik. Meanwhile, Dr. Robotnik did his best to set up an autocratic government, and legally turned anyone who stood against him into robots devoid of freewill, and forced the nobles into paying large amounts of money to him as tribute.  When Sonic, Manic, and Sonia grew up, the Oracle of Delphius revealed the prophecy to them. After that, Sonic, Manic, and Sonia decided to go on a quest, searching throughout Mobius for Queen Aleena. Dr. Robotnik, with the assistance of the Swat-Bots and his bounty hunters Sleet and Dingo, tries constantly to capture the royal hedgehogs and prevent the prophecy from being fulfilled.  The Oracle of Delphius has assigned the three siblings powerful "medallions" that can change into musical instruments, and can also be used as weapons. Sonic's medallion is an electric guitar, Sonia's medallion is a keyboard that functions as a smoke machine, and Manic's medallion is a drumset that can be used as an "earth controller" with cymbals that can deflect laserfire. All of the medallions can be used as laser guns. The three use the amulets not only to fight Robotnik's forces but to also as instruments for their underground rock band, "Sonic Underground." Answer this question using a quote from the following article:

Who are those three?
Queen Aleena separated her three children: Sonic, Manic, and Sonia