Falling fuel pressure

If I understand your post correctly, you have a pressure gauge measuring fuel pressure at the carburetor. When you start the car, the pressure starts at 5 psi and then drops to 3 psi.

How long does it take for the fuel pressure to drop and does the fuel filter also show less liquid fuel as the fuel pressure drops?

Does the car continue to run OK with the lower fuel pressure?

What happens to the fuel pressure if you run cold water from a garden hose over the fuel pump after the pressure drop to 3 psi?

Whereabouts are you located?