As of 2020-12-15, Link of ArcObject 9.3 Document is redirect to ArcObject 10.0.
How can I view 9.3 Document ?
http://resources.arcgis.com/en/help/
Given ArcGIS 9.3.x was formally retired 7 years ago, I am not sure if documentation is really the issue. Esri has been generous to leave that old documentation online for so many years, so I can't blame them for not hosting it any longer.
Fortunately for you, the Wayback Machine (archive.org) has years of snapshots of the old documentation you can pull up.
Thank you for the Wayback Machine. At least I can still access it somehow even the pictures are missing.