Conversation so far:
What was the Sonic Underground's plot?
Sonic Underground follows a main plot where Sonic was forced to separate from his mother due to a prophecy told by the Oracle of Delphius.
What was the relation between Sonic Underground and the plot?
The show takes place in a separate canon and continuity than any other Sonic the Hedgehog media.
Are there any other interesting aspects about this article on Sonic Underground other than the plot?
It was the first Sonic the Hedgehog television series to be sold on the iTunes Store.

Question: Who are those three?
Self-contained rewrite: Who are those three children of Queen Sonia in Sonic Underground?