Excellent comment.
The Manderlys hate the Boltons. Tbe Umbers would hate the Karstarks. I see no chance of a three-way alliance.
BTW — Smalljohn’s unwavering refusal to pledge loyalty to Ramsay was highlighted for a reason. They didn’t have to go there.
Yes, the only way that kind of alliance would work is if they agreed to work together against someone they hated even more.
The only one I can see they might all hate more than each other is the bastard, raper, kingslayer, kinslayer Ramsay :)