Always download from the official sites (e.g., codecguide.com for K-Lite).
Out of the box, Premiere Pro CS3 supports standard-definition and some early high-definition formats: adobe premiere pro cs3 codecs free download
To expand CS3’s capabilities on modern systems (Windows 10/11), these free codec downloads are often recommended: Importing MP4 Into Premiere pro CS3 - Adobe Community Always download from the official sites (e