In mission 3, I can't attack the dragons guarding the passage to the Dominion. Why is that? (All optional and main quests for the Necropolis have been completed).

Is it the old or new version of the campaign?

To complete the mission, you need to:

1) Complete all main quests from the main altar (Destroy all Inferno castles, complete all quests from the 4 additional altars, obtain information about the ring in the tavern, then teleport to the main altar and receive a notification that the passage has been unlocked).

2) For the additional altars, complete the following quests:
- Capture all Inferno dwellings (18 of them) and teleport to the quest giver (all teleports are done using a special artifact, the "Scepter of the Underworld." In the old version, it must be worn by the hero; in the new version, this is not necessary).
- Free the angels (3 squads) and teleport to the quest giver.
- Find 2 Necropolis artifacts and teleport to the quest giver.
- Kill Grok and teleport to the quest giver.

I tested the new version twice, and there definitely shouldn't be any problems there. And I don't think there should be any with the old version either.