The retail box version of Mac OS X Server 10.4.7 (Universal) does not support PowerPC G3-based Macs, but Mac OS X Server 10.4 (non-universal) updated to version 10.4.7 does support PowerPC G3-based Macs.
Mac OS X Server 10.4 (non-universal) install media and software updates work on G3 based Macintosh computers. Consult your Mac OS X Server documentation for information about supported configurations.