I don't have mine yet (still being shipped) so I can't provide details from SIII now.
SIII is comprised of these chapters: Sorrow, Verger, Nomad and Future & Hope.
About Verger you can read some from Cecilia Rodrigo page in the Wikia.
Future & Hope in this thread. And Nomad you can find it at the end of manga vol.3
Sorrow is newly written chapter. As far as I understand from reading atwiki pages, it happens during the American Civil War about the cooperation between
Friagne and a human,
Billy/Billie Hawkin(not sure about the translation)...
Spoiler for About 'Sorrow' in a nutshell:
who wanted his revenge on a certain Flame Haze and his commanded army that burned down and destroyed his village. At the verge of death, he successfully created, with the help of Friagne, one of the most dangerous Treasure Tools in history and named it
before succumbed to death.