It doesn't seem load P3Dv3 flight plans, but I know it can load FSX flight plans though. The 777 can't load the Captain Sim flight plans created by PFPX either. That's because PFPX saves them in the format that the 757, and 767 use, not the format that the 777 uses.

Also. The CS777 for P3D was originally done for P3Dv2, and I'm not so sire it's fully updated to P3Dv3, but everything else seems to be the same in P3D, as it is in FSX.

I have sometimes entered the route manually into the CDU, by copying the route info from PFPX. That can take a while to do on some long routes, so it's not always what one might want to do.