Guess what? IT WORKS! The only thing I had to do is disable Google in the Location services app.
I don't know why, it seems that Google location data has precedence over the GPS.
I did some tests walking yesterday night: First, the inbuilt Bing Maps app, which worked very slowly, but that's because I've exhausted this month's data plan, and I'm on glacial plain 3G speeds to download map tiles.
Next came Navit, which knows how to deal with the famous "GPS bug" that affects everything west side of Greenwich. It worked fine, except for some little degree jumps in orientation. I've had never seen this before, but it might me due to be walking, instead of riding my motorcycle or driving my car.
It also didn't crash like it usually did on me several times. Do you know why it's unable to find many streets, that were indeed found in the previous versions for the original Pre? I'm using exactly the same map files, done by myself in the fantastic Navit Planet Extractor.
Finally, NDrive came and, to my surprise, it indeed works great. I thought it would suffer from the "GPS bug", as it hasn't been updated like Navit.
All these tests were done with several other cards open, and while listening to music. All went smooth in this Pre 3, no glitch, no lag, no nothing. That's impressive, compared to how my poor original Pre struggled to just open a single navigation app, as it was permanently working in swap memory.
Those were heroic times - The Pre 3 is a champion!