I advised him to capture the nearest Castle and then head to the northwest Castle. I didn’t have any problems with this in version 1.41. But here, right before the Castle, there’s a garrison. I don’t remember if there was a garrison in 1.41, but if there was, it wasn’t this big. I definitely wouldn’t forget such a garrison – 50 Ents, 100 High Druids, 350 Archers, and 400 Dryads. It’s impossible to jump over it because, in one of the patches, the “Astral Gate” spell was removed from the Guilds of Magic Campaigns.
I suggested he start over and go to the Dungeon to capture his Castle because the northeastern city is too far away – the entire army will scatter. But he was also overwhelmed by underground heroes there. I sent him to the shop, and I tried starting over while he was gone. I had better luck – I captured the Castle, bought a Demon hero, defeated both underground heroes, broke through the outpost, and returned to my playthrough of the 5th Campaign.
Now I don’t know what my friend should do next. I played this mission differently. I think it’s better not to linger in the Dungeon. The Castle is completely undeveloped. Therefore, it takes a lot of time to gather a decent Demon army. I advised him to buy another hero to deliver reinforcements from his home Castle, and with the first hero, without wasting time, head to the northeastern Castle. He will have to send the Druids back every night.
It seems that in version 1.5, they simplified the 3rd mission but made this one more difficult.