El Capitan and old Photoshop to bootable partition
Macs very rarely support booting versions of macOS older than the one they shipped with—exceptions are few and far between, and never more than a year's release. This means that, at least on a new machine, partitioning and installing an older version on bare metal is out of the question.

Though CS3 does support Intel Macs without Rosetta, and in theory, the binaries should launch on modern versions of macOS, compatibility with newer versions is... mediocre... at best. There's some info floating around about it, but nothing is really guarenteed to work.

As it stands, your options are basically as follows:


Try to get CS3 (or if you want to up your odds, CS6) to work on a modern version of macOS
Continue using your old MacBook Pro (or another Mac) exclusively for CS3
Run an older version of macOS in a VM and deal with the potential slowdowns/issues associated with it (macOS isn't usually the happiest about running in a VM compared to other OSs.)
Use a Photoshop alternative (e.g. Affinity Photo, Pixelmator, etc.)
Bite the bullet and pay for CC


I understand that none of these options are really that great; it's something I've struggled with too, personally. However, Adobe gives no other option really.