I am in the city of Chicago. I have an old building that was once a single family converted to a 3-flat. There is a garden unit, 1st floor apartment and 2nd floor apartment. The city service is 1" lead at the meter and reduces to 5/8" lead somewhere under the building (actual I.D. is about 1/2". We connected new 1" copper to the 5/8" and branched all new copper to 16 fixtures from there. There is a separate 40 gal water heater for each unit. With nothing on, the pressure is at 36psi. The pressure and volume drop dramatically when any fixture is in use. When more than one fixture is in use, the third floor gets NOTHING!! The 3rd floor shower head is approximately 30 feet higher than the city service in basement. Without tapping 1-1/2" into the city main, what are some suggestions to rectify this problem?

the ID of 5/8" lead pipe is smaller than 1/2". probably more like less than 3/8"
way too small.
you'll have to dig to the street and run in at least 1 1/4" for 3 units.

If the third floor bathroom is 30' higher than the water service you are loosing about 15 psi.

I agree with Packy. In the City of Chicago, County of Cook, I'm sure that you pulled a permit before you converted a house into a 3 Flat. I'm also hoping that you used a Licensed Plumber to do the work and the job was inspected.

A licensed plumber would have told you the service might have been ok for a single family dwelling but not enough for a 3 Flat. I'm thinking the inspection would have confirmed that as well.

I think you will find out more when you try and sell this building......

Of course, you could build a water tower next to your 3 Flat to increase water pressure in your building like they did years ago on those 20 story buildings in the downtown area.

Good Luck with your new and future endeavor.... you'll need it.
Best Wishes