Hey Jens,
This is what I do and hopefully it can help you.
Method 1: this assumes that the STAR’s you will be using start from the same waypoint.
Create a flight plan specifically for PF3, but only include all waypoints up to the FIRST waypoint of the STAR then direct to the arrival airport. You then select the STAR names in PF3 using the SID/STAR page for your arrival airport. When you load your flightplan into PF3, you assign the last waypoint as the start of your STAR and its crossing altitude. Depending on your arrival runway, when you approach that waypoint, you will be cleared for the approach and given a STAR. You can then just follow the STAR using your own navigation source (FMC/GPS).
Method 2: This is used if the STAR’s to your airport have different entry points (waypoints) from the direction you will be approaching the airport. It’s basically the same as method 1, but you don’t include the first waypoint of the STAR in your flightplan. Make the last waypoint near the entry points for the STAR’s at your arrival airport. In PF3, make this last “common” waypoint the start of you STAR. Once again, you will be cleared to follow a STAR dependant on the runway being used.
Basically what I’m saying is that you don’t need to enter every single waypoint on your STAR into the flightplan that PF3 will use, just the first one. That’s enough for PF3 to assign a STAR depending on which runway is being used. It’s then up to you to follow the published star until you are on final when PF3 will ask you to contact the tower.
I hope this helps