I'd suggest throwing together a random encounter table that represents a disgruntled army spread throughout a forest, make them roll skill checks to avoid an encounter for every interval of travel.
They could stumble across a group of sell-swords that try holding them up, or a group willing to help them negotiate through the woods for a small fee (circumstantial bonus on skill checks, circumventing the biggest threats). I'd also have a few guards through out the woods. If the party passes the stealth check, the guard is oblivious, if they try diplomacy, maybe the guard helps them, if they threaten the guard, maybe he calls his buddies.
You probably don't want your 3 PCs trying to fight an army all at once, so just give the sense that the place is saturated with soldiers, but they aren't necessarily all threatening. Now of course, anytime morale is low in an army camp, divisive factions form, maybe the PCs can get assistance from one, while the other group is a bit more hostile? If they play their cards right, they may be able to get by with minimal harm, but if they misstep, they could bring a world of hurt down.
Are the woods well traveled with paths and roads through them? If so, maybe the bandity soldiers set up tolls. You could look into the chase mechanics to represent a platoon of soldiers running them down.
Another thing to consider is what the troops may want out of the PCs. Are they looking to ease their tension with a good fight? Would they try kidnapping them to hold for ransom, or just spilling their guts and taking their stuff?