Want to install Adobe Photoshop CS 3, Adobe Illustrator CS 3, or Adobe Flash CS 3 on a new Mac running macOS High Sierra? If you own a license, it's possible.
The copies you probably downloaded way back when required activation by a remote server that's no longer online. Luckily, Adobe will give you a new serial number and installation that doesn't require remote activation.
Visit https://helpx.adobe.com/creative-suite/kb/cs3-product-downloads.html and log in, creating an account if necessary. Then plug in your old product serial number.
Save-down your new serial number somewhere safe. Then, click the appropriate product download link. If you don't already have Akamai web installer installed (and you probably don't), then you'll get that — not your Adobe CS3 software. If necessary, complete the weird installation process for Akamai Web Installer. This should result in a new macOS control panel. If opened, this control panel will pronounce Akamai "started".
Now, go back and click that download link again in your web browser, this time with Akamai running. You should get your CS3 product download!
Open up the .dmg file, double-click the product folder, and run the Setup tool. Pick a volume to install your Adobe software on. Once installed, run the software. You'll be prompted for the new serial number. Afterwards, you may see a blank dialog box. Wait a few minutes and receive a message that your software can't be "registered". That's fine, elect never to register.
After that, you should be good-to-go!
You may want to uninstall the weird Akamai web installer at this point. Open the Akamai folder in your user account's Applications folder. Then run Akamai NetSession Uninstaller.
If you asked me, it's something of a minor miracle that these applications still work. I'm guessing it's somebody's job at Apple to keep macOS upgrades from breaking them.