Unsuspecting
They all underestimated their power.
Citizens would laugh at their antics and shake their heads while saying, "Those three are at it again!"
The Powerpuff Girls would smile condensendingly, as if the only reason that they would talk to them was to humour them around for a bit, to watch them make fools of themselves.
Even other villains figured that they would never amount to anything; every hero needs at least one pathetic enemy.
So when the day came where they, all of them, could only cough as the disease spread rapidly and deadly, the Amoeba Boys smirked, victory insured.
... ... ... ...
Okay, I know the Amoeba Boys are rather out-of-character here, but this is just one of my insane fan theories.