This is a very easy mission. To win, very quickly, after placing the granary, place a market, armory and barracks and buy at least 30 maces and 30 leather armour. Create the macemen in the barracks and send these to one of the Snake's keeps, killing the lord before his walls are up. Send them to another Snake's walls immediately after killing the first one, and after the second Snake is killed, send them to kill the third one. This should not take more than 5 minutes.