Has any thought been given to revolving conflicts with more than one side? What about if a side joins in late?
If you’re not with us you’re against us?
(Note: the following is completely off the top of my head, I didn’t know there were rules for this sort of thing in Mouse Guard… though I still don’t know what those rules are in Mouse Guard…)
I suppose if you wanted to play out a conflict that really had more than two sides you could do it this way: Each team picks an action in the normal way. The action they choose affects every other team. If multiple actions would affect you, you evaluate each action and only the worst one applies to you (If one enemy would drop your disposition by 5, and the other would drop it by 1, then your disposition drops by 5). Keep in mind that if it’s really three sided, then your enemies aren’t cooperating and they’re getting in each other’s way trying to hurt you as much as they are helping each other hurt you.
You could also take the option of choosing which enemy you target with your action, but I feel like that breaks TB more, and if you are choosing the worst enemy of the two, then what you likely have is a sequence of conflicts. One where two teams are ganging up on the third, and then if they win, they turn on each other. So I’m not sure the targeted approach really makes sense with the rules. Then again we are definitely in alien territory here.
As for new people joining in during the middle… One conflict at a time seems to be a pretty solid rule. I suppose it wouldn’t be completely insane to end combat early and give each side a compromise appropriate to their enemies lost disposition, but that would be very unconventional. Anyways, you don’t get twists until the combat is over, so where are these other people coming from in the middle of the fight?
Hmm, I suppose if the players describe themselves kicking down a door that’s known to have more enemies behind it then those enemies might join in the fight… if you get in a situation like that and you want to have them join in, the best option seems to have them roll helping disposition dice, and add 1 person with 1 disposition for each success you get from those new helping dice. So if 5 new enemies are encountered during a conflict, you would roll 5 helping dice, and if you got 3 successes then 3 enemies with 1 disposition each would join the fray…
This definitely all seems to be way outside standard Torchbearer play, so I think I’ll stick with my first answer 
A side joins in late isn’t working by the rules. It’s an excelent compromise though.
For 3 groups you could just use the Mouse Guard rules i guess.
I should really take a look at mouse guard sometime.
So the more I have thought about it, the more I think that I’m not adapting my brain to the way Torchbearer handles things, especially time. It seems to me that the rules are about facilitating drama through hard choices and resource management.
The specific scenario I had in mind when posing this question is the age old trope of failing to sneak up on or defeat a sentry before they have time to warn the next room. Whereas in the Old School game, alerting the next room means that in 1d6 turns (or however long) the reinforcements come and make life harder for the PCs. But in Torchbearer what I should simply do is ask the players what they’re doing. They sneak up on the sentry? Great, if they succeed they fight it out per normal. If they fail, then I throw in a twist that the sentry alerted the next room and I add them to the fight.
I’m still stuck on how to play a battle where 3 sides each have very different goals in a conflict, but the above suggests are helpful.