So I copied my settings from FX2 for the most part and updated the applaunch number to 442120. At this point I can get the app to launch but no game is selected or launched. Another minor issue is a few tables had their filenames changed slightly such as STARWARS_Episode_7 is now STARWARS_Episode7, but 4-6 are the same as they were... I got cabinet mode going on FX3 as well.
[PinballFX3]
Enabled=True
WorkingPath=C:\Program Files (x86)\Steam
Executable=Steam.exe
Param