The gap between those games is 15 years. Dunan's Unification war was 460 and the Second Fire Bringer war was 475. Even though S3 takes place after S2, it doesn't necessarily have to be thought of as a sequel. There are extra things you can do with a save file from S2 (even better if that one was also used with a save file from S1) but you can still get the full story without that prior knowledge.
Sent from my Ono-Sendai Cyberspace 7 - Break the ICE