It's been a while, but the uni bookstores would often carry the educational versions of software. In your case, it sounds like you've done the pricing research and found the edu cost is the same for photoshop. When I was studying at a local community college for some continuing learning in IT, the college was an MS and Cisco partner. We were able to get MS software for ridiculously low prices (basically shipping costs). Not sure if your institution might have a similar deal with Adobe - sounds like checking with someone other than your daughter's instructor might be worth it - a counselor or adviser maybe?
For what it's worth, Photoshop CS3 is probably 90% the same as CS6. However, there are a few killer features that are missing from CS3. But in an intro course, I doubt that those features are going to be used. Most of the instruction will probably focus on the basic tools that have been in place for a decade or longer (my wife teaches a class that uses photoshop). I would, however, anticipate that any instruction may need to be modified slightly for CS3 - similar to what one might have to do if they were using a previous release of a text book. I believe adobe offers a free 30 day trial of photoshop, so that may also be an option to check out.