Vent a portable air conditioner with a 6 inch hose through a 3 inch hole in the wall
The reduction of the diameter from the 6" of the hose to 3" through the wall may not cause a significant problem. The resistance to fluid flow of a pipe is [inversely] proportional to the square of the diameter and directly proportional to the length of the pipe. So if the length of the 3" diameter hole is short compared to the length of the 6" hose, then the constriction to 3" may work OK. You could ask the mfgr, but you may have to try it. I bet it will work fine.

The key to making it work would be to make a gradual transition from 6" diameter to 3" diameter over say 8 inches to 1 ft of length. This would prevent the creation of turbulence which could restrict flow. Presumably the 3" hole in the wall is lined with a pipe, right?

I wonder if those 3" diameter holes are positioned vertically so that a ductless mini-split a/c could be installed. How high above the floor are these holes? How far are they from the ceiling? Would the building management allow that?