Drawing a system curve for pumps running in parallel

Hi all

Can you help me how to plot the system curve for centrifugal pumps running in parallel operation.

Well, here is the case,
I have three pumps with nominal capacity of 1440 m3/hr. the line is starting with two bellmouth elbows from two different tanks, each bellmouth is connected to a butterfly valve then a dismantling piece then the both lines are connected to the suciton header. all the previous lines are DN900 GRP pipes. out of the header are three lines 600 DN each. each line starting from the header with DN600 then a reducer to DN350 then to its pump, out of the pump with 350 then outlet expansion to DN 500 then to the discharge header DN 800 then to the discharge line DN 800.

How I draw the system curve for such network. what will be the flow for my calcs, shall I calculate to only one pump with its nominal flow, or to one pump with the full flow rate, or what.

Your kind reply, will be highly appreciated.

Best Regards