Is there a way to configure PF3 so ATC doesn't manage my flight level? I just want to use the autopilot and let the plane computer, fly the STAR and the approach.
Not really. You will not get a 'descend via the STAR' instruction. ATC will step you down according to the altitudes entered on the PF3 waypoint page. In real-world situations descending via the FMC often isn't possible and ATC needs to give descent instructions due to traffic below you.
However, you can cheat a bit and adjust the altitudes on the PF3 waypoint page to roughly correspond to the altitudes in your FMC, so PF3's instructions will match your FMC's descent profile. I do this sometimes and it's possible to ride the FMC down to the altitude for the last waypoint. For example, if your FMC has you at 11,400 at a waypoint, enter 11,000 for that waypoint on the PF3 waypoint page. Caution: You may need to adjust altitudes of nearby waypoints, including XXX waypoints on the PF3 waypoint page. Hover your cursor over the waypoint names to see the distances to adjacent waypoints.