A K3 civilization is one that has diverted a significant fraction of the stellar output of a galaxy to its own uses. They have to dump their waste heat somehow, and that's visible in the far to mid infrared. So we can compare the IR emission of galaxies to look for oddballs with a lot of IR.

https://www.centauri-dreams.org/2015/04/16/g-hat-searching-f...

“Our results mean that, out of the 100,000 galaxies that WISE could see in sufficient detail, none of them is widely populated by an alien civilization using most of the starlight in its galaxy for its own purposes. That’s interesting because these galaxies are billions of years old, which should have been plenty of time for them to have been filled with alien civilizations, if they exist. Either they don’t exist, or they don’t yet use enough energy for us to recognize them.”

When we assume K3 capabilities, why don't we assume they dump their waste emissions away into something like (artificial even?) black holes? Maybe they are shy and afraid of the intergalactic "Dark Forest"? ( https://en.wikipedia.org/wiki/The_Dark_Forest )

The basic problem with that idea is black holes are too small. The amount of low temperature radiation that one could dump into them would be insufficient.

I think the point is that if a large number of galaxies had Type III civilizations, we would notice, because the definition is that they use most of the available energy in their galaxy making the galaxy radiate more infrared than visible light. But that doesn’t mean life isn’t common, it may just mean that a Type III civilization is either physically impossible or very unlikely.