Designing a PID controller for a pump station

Hello all, I have a pump station with 3 pumps and I have to control the speed of the 3 pumps to maintain a desired water level in a tank. To do that i have to design a PID controller. I can't obtain any data from the pump station so i can't identify my system i need to model it, and i don't know how to do that. If you can help me with that or send me links that my help it would be nice.
I have sat in meetings where the Process Engineer/client have no idea how a piece of equipment will perform within their process. So you put forward proposals only to be told that will not work but this will....
and you find out what they really have in mind. So how do you know PID control will be compatable with your 3 pumps, with no information ?

Some time I had a scenario of controlling pump speed from water level in a wetwell. But at least I knew the 3 pumps each had 70kw variable speed drives and the configuration was Duty, DutyAssist and Standby. It was only at commissioning did we find that slowing the speed down below 64% of maximum, pump flow stopped, the impeller merely spinning serving no useful purpose. At that point PID control was automatically switched to manual preventing pump speed falling below 64%; this being the only complication of the (PLC based) controller.

The complicated part was the logic determining which pump was duty as selection was automatic, so after several days each pump had a turn at being run for several hours.
You are quite a ways from needing to design a PID controller! In fact depending on the control system to be used you might not ever need to design one, you’ll just select one. First, it sounds like someone else, a process engineer possibly is designing the system? What are the signals you are given? What are the performance requirements, what is the operating scheme for the three pumps?