If you still want to use CS3, I would suggest to wait on upgrading to El Capitan. Wait until everybody is happy.
I have an iMAC and have been running CS3 on Yosemite (with a plug in I downloaded long time ago). It's been fine, perfect. All that until I upgraded to El Capitan. Now none of my CS3 applications open. In Photoshop, I got the message, "Adobe Photoshop not open in this computer". I've been in every website looking for answers.
I'll go on details:
I upgraded to El capitan on Saturday (I shouldn't). Right away, upon trying to open Photoshop CS3, it asked me to first download Java 6 to keep legacy. It worked fine until Sunday. I was working on a project but had to leave, so I shut down my Mac, came back about an hour and a half later, turn on the Mac to continue my work and Adobe Suite CS3 did not open anymore, exactly 24 hours after I upgraded to El Capitan.
Searching for answers, I found out I was not the only one with that problem. Many users are even having problems with CS6 !! One solution on the Apple community suggested to downgrade to Yosemite. For that I need first to erase El Capitan. I started to follow the steps but got stuck on the 2nd one (no alterations on the Mac yet), when one of my windows did not match what the step-by-step solution explained. I'm not too good at some technical stuff, I know my limits, so I decided to stop there.
I also know there will be a fix coming from Apple. They always help, I'll just have to wait.