Well, the Borg did stop short of assimilating Icheb's home - because they saw that assimilating bits of it while letting others regrow provided them with more to assimilate. And they have been around for hundreds of millennia, but haven't overrun the galaxy yet. So "holding back" is probably at the very core of their programming...
Replicators tend to suffer from a shortage of FTL starships, but otherwise they expand more aggressively than the Borg, and would probably go exponential as soon as they gained sufficient transportation capacity.
Never heard of the third player. Man of Steel gets silicon implants?