If you want a free alternative, Adobe offers (limited features) or you can try GIMP , Photopea (browser-based), or Paint.NET .

This article explores the history of CS3, the reality of finding it for free, the severe risks of downloading cracked software, and the best legal, free alternatives available today. The Legacy of Photoshop CS3 (Version 10.0)

If you legitimately purchased Photoshop CS3 in 2007–2009 and have your original serial number, you may be in luck.