Markoz,
Now that I look again, yes it does look like the -300 module I purchased is NOT advertised as for P3D.

But in spite of that, it appears to fly just fine in P3D. The -300 autopilot also appears to work in P3D.

Some key combos (shft-5, shft-6, and shft-9) just show a black rectangle so as you suggest the -300 doesn't look to be 100% compatible.