From the Knowledge Base article:
Cumulative Update 1 for SQL Server 2005 Service Pack 3 contains only hotfixes that were released in Cumulative Update 10 for SQL Server 2005 Service Pack 2 and Cumulative Update 11 for SQL Server 2005 Service Pack 2. These hotfixes are not included in SQL Server 2005 Service Pack 3. CU1 for SQL 2005 SP3 is only intended as a post service pack rollup for SP2 CU10 and SP2 CU11 customers who plan to upgrade to SP3 and still keep the hotfixes from SQL 2005 SP2 CU10 and SQL 2005 SP2 CU11. No new hotfixes have been included in this Cumulative Update other than the hotfixes that were already released in CU10 and CU11.
So.. if you need it, go get it!