Update:
The Migration Assistant didn't help me, because it didn't allow me to install just CS3. I would have had to migrate all applications!
As it turns out, the installer that comes with CS3 is apparently in Universal Binary. I could simply install CS3, and from what I can tell so far, at least the core funtions are working fine in 10.7, so I'm happy. Droplets cannot be created, because those were designed for PPC (per a pop-up window).