I've never worked with Photoshop CS3 (I'm still using 7), but Photoshop Elements is basically a scaled down version of the full Photoshop aimed at photo clean-up/manipulation. Color selection is simplified, you can't export to CMYK and many tools/plug-ins are either simplified or eliminated entirely.
By the way, it should be pointed out that Photoshop Elements came out with Photoshop 7, so it may not be comparable to CS3 features. I do not know if they released a new Elements to coincide with the latest Photoshop. I haven't been keeping up so someone feel free to correct me.
Your best bet would be to download a trial of Photoshop Elements and see if it can do what you need it to do. Depending on how detailed/complex your CPO would be, it might get the job done. It's hard to say for certain without knowing exactly what it is you're planning on putting on your CPO.