I’ve completed the first three provinces now and can add information on the above:
Like Poison Mist, Magic Night, etc. stages in S2, certain S3 stages have a Realm active. This applies the realm bonus on all enemies in that stage, including the boss. So far it’s only Midgard realm bonus stages (since this is where the first provinces take place).
Additionally, there are stages with Elite Enemies, just like in S1! The elite enemies are new, and they feature one of the S3 heroes as the boss. The Boss of the elite enemies stages have all had a realm bonus – despite the fact that the enemy waves do not.