Hi, attached please find a presentation file from Agilent (you can also find it at the agilent web site). Using an isolator before the power combiner is important because some power from one of the signal generators may be coupled to the other signal generator and may cause inaccuracies. Of course, you have to calibrate your measurement setup first. Especially the losses at the output of your DUT is critical. I don't know what kind of an amplifier you measure, but ip3 makes sense for low noise amplifiers. As IP3 is obtained from an extrapolation, your input power should be small enough.