U need to do it through the Sweep option (Present at the end of PSS analysis form)in the PSS Analysis if you want exact answers. Use port instance for your input signal from analogLib provided....
Use the power in dB option available in the input port instance.....give a variable there and specify tht varible in the swept varible, in the PSS analysis....suitably choose the input power range....make sure tht the input impedance of your mixer is equal to port 's source resistance....
After the simulation you can directly observe the result from the result options available in PSS analysis Direct Form options..
Choose carefully your fundamental and third harmonic with handwritten equations.....