Actually you do not need the Thermal Package Impedance component in this case and can remove it from the three PM leg subsystems like so:
Each of the six IGBT/diode pairs are all connected to the heatsink via their individual Rth_ch interfaces.
The Thermal Package Impedance is an advanced option and is only necessary if you are using Thermal Package Descriptions for the semiconductors (this isn't the case for these Semikron devices) _and_ you include in that description a package impedance state-space system (such as to describe the interdependent heating between the IGBT and diode in the packaging). Here is a screenshot of that interface just for some further context:
One other thing I noticed in your model is that you are averaging the instantaneous switching and conduction losses with multiple averaging schemes- you should use only the Periodic Impulse Average for the switching loss as the Periodic Average block is intended for averaging conduction losses.
Anyway, I hope this helps, but let me know if you have further questions or doubts.