When with a faction, I largely ignore marshals. Generally I wait for him to give me a quest (cows or scouting), fulfill it and *do not* talk to him. This way I do not have to follow him and I can wage my own war. Also, there are ways of controlling AI army without being a marshal.
For example you can take a castle, but you can't really reinforce it alone, so you leave one wounded looter in the garrison. Enemies can't just take it, they must mount a siege and because defenses are so weak, they will try to do it over and over again. You can farm weaker lords for honor or relations and sooner or later your marshal should appear near by to defend a besieged castle. Then you can talk to him and suggest a better target. With decent relations he will follow your advice.
Don't forget that even with a very weak army you can influence the outcome of major battles. AI tends to be overly cautious, but they will join a battle in progress. If you are smart and manage to split the enemy a little, then attack, you may be able to create a situation where most of your army is facing only a fraction of the enemy forces. It works best at night.
Of course if you are already considering striking out on your own, it may be a good time. But in case you don't feel like it yet, it should be possible to win against three factions. To do that you must destroy the majority of enemy forces to the point that they will resort to raiding instead of conquest. Then you ignore beaten foes and concentrate on the strongest enemy. Be aggressive and take their land. If they take it back, no biggie, just retake it over and over again.
I don't start on my own until I have high engineering and companions levelled up to the point where they become effective trainers. Medical skills are also quite crucial. It's not a bad idea.
Good luck.