it sounds like you need to replace the pressure regulator valve with
one that operates in the 20-30 pound range - rather than the current 12
pounds in order to insure water to 3 stories.

did you pressurize the expansion tank after you replaced it?

If not, you should to maximize expansion capacity.

Have you or can you validate that your gauge is correct?
If not, you should.

Since you want to have about 25 pounds of pressure, your gauge should
have about of a 50-60 pound max, so that the working range of the gauge
is in the middle.

the water that enters the system is probably cooler than the temp after
cool down. Therefore, the pressure won't drop all the way down to the
level that it reached with the cold water used in the initial fill.