Typically, the manufacturer gives curves for each stage and you add them together in the direction of head at a given flow. In many multi-stage pumps, the first stage impeller is different than the others. If the individual curves for the impellers weren't available, you could simply take the head at any flow and multiply it by 0.667 if you can confirm that all of the impellers are the same diameter. Since each impeller passes the same flow but adds to the head pressure, you can multiply and divide to get a result for any number of stages.