I have now tested it on my Pre 3, and you are quite right, it does auto dim - I would never have noticed it, if not for this forum!
But in my opinion, it do not get bright enough in daylight, and not dark enough in darkness :-)
I have always used brightnessfix on my original Pre, because I did not notice the autodimming... ;-)
Brightnessfix does work - sort of - on the Pre 3 also, but it does not turn the pre off when sat on the touchstone, like it did on my original Pre.
I am hoping for a fix for this :-) JAson ? :-)