For 3D polar plot it will never ask you to define theta and Phi because it is 3D polar plot which will be plotted on all theta and phi. However you can find the gain in the brodside direction by observing the 3D polar plot which will be against particular theta and Phi..
For 2D radition pattern the value of theta and phi depending on what plane you want to plot? e.g. for xz plane theta= 0 to 360 and phi =0 yet i say that it is depending on choosing axis whenever u designed..
2D radiation pattern is nothing but seeing 3D polar plot in one plane .,you can realize that in HFSS itself..