if theres 3 guys in a dual, and one of them has all the resources and the extra castle is it dishonorable to gang up on him

If there are three guys in a duel you have a real problem already, duels only hold 2. You must mean a campaign.

In campaigns, if one player is overwhelmingly strong and others attack from multiple places it is less looked down on than attacking a defenseless person with multiple forces. In some cases the person will even invite more than one player to attack.