We have a 15 day money back guarantee, so you can definitely get the money back if it doesn't work for you.
We only say we support versions of dreamweaver that we test with extensively, and we have our work cut out for us testing in CS6, CS5.5, CS5, and CS4. The reality is that it is very likely that it works in CS3, we just haven't done extensive testing so we won't make any promises.
Feel free to give it a try. We do actually create the extension so that it will install on CS3 for those who haven't upgraded.