I have a small handy man service that has been asked to supply a water line from the parish (county) main supply and run it approx 3000 feet to supply some home sites. I am guessing there will be at least 2 and possibly 3 family homesites at the back end of this property. I would guess that each house will be no less than 3 bedroom and 2 bath houses. I would like to know the correct size of pipe that I would need to run in this 3000 feet before stubbing down for each home? Also If buried, would schedule 40 or 80 HD PVC be needed?

Next topic: I may run a natural gas supply line in the same trench, so will also need to know what size of galvanized pipe should I run in that same 3000 feet?

Until you are a licensed plumber and are insured and bonded, you have no business doing any of the work.

i'm guessing you will have to use anywhere from 6 inch to 10 inch pipe.
you will need to provide fire hydrants at locations that must be determined by an engineer.
as for the gas line ??? forget about it.

At least 2" for the water. You CANNOT use galvanized pipe for the gas if you are going to bury it.

Knowing the local codes in your area is paramount, before you begin the process. Considering the dollar value of your work and what is expected of you.... I sincerely hope as others have mentioned that you are certified, insured, and bonded. Otherwise, you do a disservice to those who may hire you as a handyman, thinking that you are insured, bonded and a certified and licensed professional.

Best Wishes