<>A good rule of thumb is that a group of PCs in decent shape will generally be able to defeat a group of NPCs of similar size and gear, as long as they have some MP to use. In combat, the enemies' level of Strength is also important to look at - a group of Enforcers can be hard to beat even for battle-hardened PCs<>
I usually go with this.
So with my group in mind, three players with Str 4, 4, 3, a normal encounter would be around three enemies with a combined strength of 11. If the threat roll includes several ☣, then I usually equip the threat with some (better) ranged weapons, or add another guy to the threat, or increase the strength of the threat (but keep the number) or add scrap armour to them. I usually add something extra for each ☣ I rolled in the threat roll above the first.
This can seem harsh, but the players should feel that this is a very dangerous place and nothing comes for free. They can always (hm, usually) flee or attempt to talk to them instead and struck a deal.