If you manage to complete the way you chose too then Istiana / Arzagos will make a bigger move and you will start war against more than one kingdom at the same time.
And you will not be able to make peace with them until you made them weak by capturing their big settlements.
(Making field battles, capturing castles or raiding villages gives so little advantage, you have to capture settlements/cities asap otherwise you I bet no one manage to fight with 3 well growed kingdoms..)
(Yes game selects most powerful kingdoms to fight against you at the same time, it is really crazy..)
By the way these are 3 seperate quests, you dont need to end all 3 kingdom same time, you have to focus on one and only defend against others.
(Which is I'm doing now and I'm close to end Khuzait, in the meantime Vlandia and Aserai took some of my castles unfortunately but I'm defending cities well bcz they are most important.)
It is really hard and I cheated a bit to be honest, otherwise I never could be able to manage to come this further. (First time I'm playing M&B games)