Strange heating piping.

I have this job, the heating water is piped from another building and loops through this building. A separate pump pulls the water off this loop and sends it through the building. Return comes back to the loop. Only thing is the Supply and Return take off are only six inches from each other. Upstairs supply pipe splits, and goes to the rear VAV and front VAV. Return only comes from front of building. I looked at the first VAV, supply comes off the front supply pipe. The return goes to a separate return that follows the supply and return going across the ceiling space. In other words there are three pipes??? Ever seen anything like this? BTW they want me to right up a control sequence, including replacing three way valves with two way, with new Frequency drive pumps.