how my serial number ended up in someone else's hands.
My hunch is that the serial number had already been bought and registered by someone else,
Without more info to go on, I suspect you bought a used, already registered copy.
Does CS3 work on current Macs?
I believe it does.
System requirements | Photoshop
Mac OS X Mavericks (10.9) compatibility FAQs | CC, CS6, Acrobat