I never heard of measuring IP3 of a PA.. usually it is the P1dB compression that is most important, because unlike linear small signal stages (where its IM3 is 3:1 almost always from its fundamental), in PAs this ration is not 3:1 (the IP3 curve), and it is power level dependent; i.e. the IP3 line is not a straight line, so there is no point in talking about IP3.
Concerning ADS simulation, you did not specify what you component you are trying to simulate? is it a built in theoretical model of the PA (a generic PA module found in the ADS library), or are you trying to simulate an ACTUAL PA circuit that you entered in the schematics (including devices models..etc)?
Please be more specific...