Low water pressure?
Gravity causes the pressure to drop the higher you go..
The water in the vertical pipe below the 3rd floor is putting 10lb of downward pressure against the source.
It takes 10 psig to push a certain amount of water through the pipe from the main valve to your apartment. It is essentially "friction", and the smaller the pipe, the more the friction. Now, your faucet probably has an even smaller hole in it to limit the amount of water you can get from it. Environmental regulations. So your pipe may be capable of delivering more water than the faucet will allow. If you think you do not have adequate water, measure the volume and time it. You should be able to get capacity (like 5 gallons per minute) and then see if you can fill a 5 gallon bucket in 1 minute. Find out where the flow restrictor is in your faucet and remove it. Sometimes it is the aerator on the end of a sink faucet. Take it off and see if you get more water. Also, be suspicious of the plumber. He gave you a standard number for adequate service, which means he doesn't want you fighting with the landlord over something that he told you. And the "hydrostatic head" of water is 0.5 psig/foot, so the difference in the 2 hydrostatic pressure is 20 feet in elevation, which does not sound right for 3 floors. Measure the height between the points where the pressure readings were made.