Have you tried playing on 200% difficulty on a small map? When the nearest castle is 1-2 days away? When it takes a month and a half to determine who is the strongest? I suggest we discuss tactics, castle and hero choices when playing under these conditions.
I think this is one of the most challenging aspects of Heroes 3. Yes, I have played with 3 castles (at different times and in different scenarios, of course :)) on an S map with dungeons (for Conflux, Necropolis, and Tower), but I still managed to achieve victory. By the way, the time frame you specified doesn't seem quite accurate to me – if you win, it will be faster than a month... but to win, you have to try hard ;)