I need an activation code for CS3 [CS3 is dead]

You're out of luck.  CS3 is dead. The activation servers are gone.  Activation is impossible.

Creative Suite is no longer sold by any vendor, retailer or country.

MODERN OPTIONS:
Photoshop or Premier Elements 2021 (approx $100, no subscription needed)
https://www.adobe.com/products/photoshop-elements.html

Creative Cloud Photography Plan -- approx $10/month for 12 months. Includes the very latest versions of these great products & services:
-- Photoshop CC and Photoshop on iPad
-- Lightroom on desktop, mobile and the web
-- Lightroom Classic
-- Portfolio website + hosting
-- Spark with premium features
-- Bridge
-- 20 GB cloud storage (upgradable to 1 TB).
https://www.adobe.com/creativecloud/photography.html

Creative Cloud All Apps (31 + apps & services) and Single Apps Plans
https://www.adobe.com/creativecloud/plans.html

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Beware of fake Adobe reps who may contact you privately offering to sell you software or take control of your computer. It's a scam, run away!
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3 Easy Ways to Identify Genuine Adobe Staff
https://tinyurl.com/10791730