I purchased a used 2002 Buick Regal LS in SW USA with 89,xxx actual miles on the odometer. I had the engine oil and filter changed and the mechanic told me the transmission ATF was very dark and should be serviced. So I had the transmission pan dropped and replaced the transmission filter and added Dextron 3 to bring it up to operating level. The transmission upshifted very smoothly when cold, but after the ATF warmed up it developed a harsh upshift. I had the transmission coded and it came up with a P1811 code. My Buick dealer that this refers to a faulty Pressure Control Solenoid. He referred to GM Part # 10478146 as a replacement. My question is there another PCS manufacturer that has a better track record than the GM PCS? And is it OK to mix Dextron 3 with a synthetic ATF? Thank you.

Assuming the ATF was still original i believe it already had Dex3 in it. So i belive all you did was mix Dex3 with Dex3

Some times it is not a broken PCS but a sticky gummy varnished one.
Same goes for the overall valve body/check balls in there.
Here is what you can try to eliminate P1811 Max adapt long shift code.
First off add a can of TransTune, this will clean things out well, say bye to varnish & hopefully better shifts when hot.....
Trans Tune | Sea Foam Sales Company
Next after driving 1000 miles with the trans tune in....drop the pan again, use a GM filter, and install a shift kit by TRANSGO!! Installs into the accumulator accessible inside bottom of transmission pan.
TransGo | 4T65E: SK 4T65E

This should provide tighter shifts when hot by passing the need for removing the valve body to change out the PCS.
Refill with the modern synthetic Dexron VI (6)
done with problem free shifts.
https://www.gmforum.com/buick-172/97...shifts-304350/

Trans tune, oil and filter change, shift kit and Dexron VI is a very good approach to getting the best out of your transmission. At best it may fix the problem, and at worst it will delay a failure.