The Mac version of Photoshop CS3 is different then the PC version of CS3 your PC serial number could not activate the Mac CS3 version so it does not matter that you can not download it. If you follow the download instruction you may be able to download what you want here, Download Adobe CS4 and CS3 Free Trials Here (incl. After Effects) | ProDesignTools
Thanks for that. I didn't understand it all. Tiny mind. I've installed a 30 day trial of CS4. I see that I can buy it for about £180= per year!. Not for me, a retiree.
I suppose what I can see now is a possibility is to ask Adobe for a cross platform licence for my paid up CS3. Will that work?
It would have to be a cross-upgrade to CS6 Mac version. CS3 and CS4 licenses can no longer be purchased in any form, and platform switches were always current shipping version only, or by upgrade to such.
You can download the Mac installer, but that's no help since you don't have a valid CS3 Mac serial number and no way to get one.
All that said, the Photoshop/Lightroom CC photography bundle is still ridiculously cheap. Yes, it's subscription, and yes, the price may go up some time in the future. But it's an option (and unlike perpetual licenses cross-platform).
Platform swaps only apply to current versions:
As twenty_one said, the photography bundle would fit your needs, is affordable and avoids al lthe technical issues with compatibility on contemporary operating systems that you face with CS3.
Thanks for the advice. I now understand the position. I have already migrated my up to date version of Lightroom 5 which is my main Adobe, app as a photographer. That was no problem and I now see why thanks for your help. My questions are answered.