HAZUS MR-3 Patch 3 was released but remains unavailable until FEMA either posts it on the official FEMA download site or OK’s another entity, perhaps one of the vendors to make is available for download via a NON-FEMA FTP site.
The Patch 3 is approximately 500 MB and has been ready since April 20,  2009.
The Patch requires that HAZUS MR-3 Patch 2 already be installed prior to installing Patch 3.
Patch 3 for MR-3 makes HAZUS compliant with ArcGIS 9.3 SP1 and is being tested on a newer version of ArcGIS for conformance.
Major fixes to the Flood Model and compliance with ArcGIS 9.3 were the noted enhancements for the MR-3 Patch3.

HAZUS MR-4 is anticipated to be released before or at the Annual HAZUS User Conference in Raliegh, NC in August.
MR-4 will add Risk Assessment Modeling for Hazards associated with Levees and Dams along with other enhancements.

Don’t forget to get Patch 2 first.

For information and download instructions regarding HAZUS-MH MR3 Patch 2, please visit: http://www.fema.gov/plan/prevent/hazus/hz_patches.shtm

With this patch release, HAZUS-MH MR3 will become certified to operate on either ArcGIS 9.2 SP5 or ArcGIS 9.3 SP1.
The release notes were in error.  There are elements of the flood model that do NOT work with patch 3 in XP SP3.