After getting my P9P and then my PW3 about 1 week later I took the following steps to get my PW3 to work with my P9P-GOS.
I waited to flash GOS on my P9P until after I received my watch - so I could setup my PW3 against stock android. This was not necessary... and created a LOT more work for me in the long run. I thought I'd read that this was a necessary step - especially for setting up Google FI as a carrier on my PW3.
After setting up my PW3 against my P9P-stock, I was amazed how much I disliked stock android... The amount of 'noise' it generates along with the associated effort to reduce that noise was suprising (after about 3 years on my P6P-GOS).
So decided to flash GOS on my P9P - which was very easy and smooth - no issues.
However when I tried to pair my PW3 with my P9P-GOS, I could not get it to pair.... tried a number of tips I found online but nothing worked.
... So I went back to Stock Android on my P9P - (unrelated but I had issues going back to stock, turns out the laptop I was using was having some space issues... so when I switched to a different laptop - that resolved.
HOWEVER - I still could not pair my PW3 with my P9P-Stock. Turns out that once the PW3 is paired to a device it must be reset (hard disconnect/reset) - and erased. Then and only then can you pair.
After pairing my PW3 back with my P9P-Stock (and still hating stock android), I decided to reflash GOS on my P9P - and then re-pair my watch after another hard reset. That did it.
I am now on my P9P-GOS with a fully functional PW3. Works great.