reptile
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http://heroes.ag.ru/h3/campaigns/AB/5/ This website helped me with completing some campaigns (not just the third, but also the fifth). I haven't read about this specific campaign myself, but I understood that you need to level up your heroes so that at the beginning of the last mission, you have around 6 heroes at level 20-30.
P.S. I tried to complete this campaign myself, and it's difficult (at least for me). I couldn't complete the second mission, and the save files are long gone, and I don't feel like starting the first mission again right now.