If they let Toph get defeated, I will lose so much faith in the show. Maybe idealism or dumb hope but Toph is a master, and a good chunk of the time unstoppable. If they end up bringing the Big Three into the battle only to be defeated, I will not be happy. To have mastered bending at a young age and able to hone their skills even more over their lifetime to only be defeated in the final battle would be a horrible way to end the season.
In contrast, I can honestly see a parellel to the first season where multiple groups attacked from multiple sides. I think the Big Three could come in and dessimate the army while Korra takes on Kuvira head-on.