http://heroes.ag.ru/h3/campaigns/AB/5/ This site helped me with completing some campaigns (not just the third Heroes game, but also the fifth). I haven't read about this campaign myself, but I understood that you need to level up your heroes so that at the beginning of the last mission, you have about 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 saves are long gone, and I don't want to start the first mission again right now.