OP doesn't mention what his missus will be using it for or her experience level. Very generally speaking, the biggest advances have been in Photoshop's context-aware tools and editing features whilst in Camera RAW- which not everybody necessarily has a need for. For non-astro work I'm still perfectly happy on CS6 and that's coming from someone who used to (but not anymore) use PS professionally. To clarify I've never done any kind of astrophotography - I'm purely visual. Certainly if her experience level is low then CS3 will more than suffice for now. That's because the PS learning curve is steep and it's not an intuitive piece of software to randomly experiment by pressing buttons.
On a side note, for anyone who is familiar with the PS interface already, and wants a free online alternative - without having to learn a new UX - then head over to www.photopea.com which has been going for a good few years now. The 'Open' and 'Create New Project' interface will be an unfamiliar mess, but once into the editing stage the interface is 100% pure Photoshop from yesteryear.
Also, if your CS3 happens to be a Mac version, you'll probably find it won't install on Catalina or more recent macOS.