If anyone has paid the small fee to use REX Essential Plus Overdrive with both FSX and P3d V2.X, here's some good news - It will work with V3.
I have a number of old programs that I haven't tried to use with Prepar3d V3.
Last week I tried REX Essential Plus Overdrive to see if it'd work. All that it needed was to be redirected to the V3 location and it all worked.
I hadn't made any changes, or so I thought. But obviously Rex did.