The H.264/MP4 capabilities in Premiere Pro CS3 were licensed from MainConcept. In a legitimate CS3 installation, these are included. If the export options for H.264 are missing, it typically indicates a corrupted installation or missing registry keys. The "free" solution is simply ensuring the software is installed correctly via the original installer.
I understand you're looking for information about codecs for Adobe Premiere Pro CS3, but I need to provide an important clarification and some helpful guidance.