Finally I did give Level 3 a fair shoot and well turns out to be okay to play. Nothing special, just push to the yellow borders and attack asap. Some minutes can be saved if the yellow player already occupied the Portal, so you just have to conquer the buildings. It does not happen often and the thing is the faster you play the more unlikly it gets the Portal will be within the yellow borders. Anyway, its a good level for beginners, cause you need a certain build order and a good road system to keep the stuff flowing all the time.

I played the Settlers 2 Gold Version [from gog] played on DosBox