Help my identify all the solder points (not the gpio points)on the back of the raspberry pi 3 model B please. this would help me a lot and I'm sure would help others too.
I'm guessing, they'd prefer people not to use them, because they could change from version to version, whereas the 40-way connector and the other connector have a more fixed pinout. But as you say, I'd prefer that it was published for maximum flexibility along with a notice that they would be highly likely to change in the next version Pi. The risk of using these pads has been highlighted in a recent Pi zero kickstarter, which was designed to connect to certain pads with spring contacts, but the positions changed from the first and second version Pi zero.
The raspberry pi foundation has published only limited schematics, so only some of the test points are understood with certainty. The documentation only contains a subset of the entire circuit. I presume you've already checked the limited schematics documentation. If you're trying to confirm the audio/video pins as marked in your photo, one way (if it isn't on the schematics) would be to insert a jack plug (with the Pi unpowered) and just use a multimeter to 'buzz out' the connections.
Please publish this information if you feel it is useful (it will invalidate warranty to solder to the underside of the Pi, so not everyone will use this method).