How Cabin Pressures Are Selected? The pilot puts the cruise altitude to the FMC and the plane auto adjusts the cabin pressure. I know that but I once heard the cabin pressure is set to three thousand feet.

Is it true? Why is the cabin pressure set to 3000 feet? How does plane designers of Boeing/Airbus/Bombardier etc. choose which altitude the cabin pressure will be set to?
As a generic answer, the only setting the crew inputs in most pressurization systems is the destination field elevation. This determines the pressure schedule on descent, and the system will use a cabin altitude descent rate that will put the pressure differential to near zero, usually slightly positive (that is, a hundred feet or so below field elevation) upon landing before the outflow valves are opened fully to vent the cabin to atmosphere.

So if the crew sets the field elevation at 3000 ft, the system will use a pressure differential reduction schedule on descent that gets the pressure hull close to zero differential when at 3000 ft, so there is minimum effect on people when the system goes into dump mode to vent the cabin.