I have the CS757 working quite well in P3Dv3. Same old bugs with the FMC and the flight model. I did have an odd issue with TCAS until I found that the Avatar Mode in P3D causes an issue with it. To resolve I disabled Avatar Mode and restarted P3D then the traffic alarms went away and the TCAS worked properly.

I think you could enable Avatar Mode after you startup P3D and it won't conflict with the CS TCAS gauge. That's how I discovered the cause of the issue anyway.