Bandwidth tuning using control system designer

Hi, I am designing a control loop for a sensor. I did an impulse response in hardware and identified the plant transfer function as a 3rd order system. I tried using PI controller tuning (pidtool) and the software is returning an integral alone controller with a very sluggish response. I tried manually using different combinations of P and I but my target bandwidth is not achieved. I tried Controlsystemdesigner and added compensators with optimization tuning but it's always returns unstable loops. I have introduced a notch due to a peaking in my response, integrator to remove steady state error. If I add 2 nos of real zero in which one is close to origin, the bandwidth is improving. I am not sure whether its the correct method of control design or implementation. Please help me with the same. Thanks in advance. Karthik R