For Mac Sierra or higher, see this help article.

Installing Creative Suite on macOS 10.12 (Sierra)

https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html

CS3 is very old software.  Adobe retired the aging activation & registration servers.  Assuming you still have an older OS to support it, you will need to obtain a replacement installer and serial number that does not require activation.  Also CS3 is not supported anymore so after installing the replacement version, enter the new serial number and check the box for "never register this product." 

https://helpx.adobe.com/creative-suite/kb/activation-fails-cs3-acrobat-8.html

Adobe shut down the activation servers for cs3, so you need a new non activation cs3 from here:

https://helpx.adobe.com/gr_en/creative-suite/kb/cs3-product-downloads.html

As far as I know, CS3 is no longer available... if you have your original serial number you need to contact support to obtain the NEW download and special serial number that works without the activation server, which has been retired

IMPORTANT NOTE:
The process below WILL NOT WORK if you have a (CS6 or earlier) serial number for an education or volume version, since the only downloads now available are the retail installers, which DO NOT work with an education or volume serial number
.
Go to https://www.adobe.com/about-adobe/contact.html
-click on the Chat bot on the bottom right hand corner of the first screen
--the chat bot is the circle with a funny looking box with 3 dots inside
-click on Downloading or Installing, then type Agent
.
Do NOT let the support agent tell you to come to the forums for help... no user in a public forum has access to Adobe's internal links, you MUST obtain the program and update links from Adobe support
.
AND when you finally do have a download link, be sure to copy every file you download to an external drive (or 2 or 3) so you never have to hunt for download links if they are moved again... also make a text file containing ALL of your serial numbers and copy that file as well