Sage recently announced they have successfully tested Sage ERP X3 with SQL Server 2012. Yes, Sage ERP X3 supports SQL Server 2012.
Here is a list of the various Sage ERP X3 V6 component levels you need to meet the support criteria.
One item of caution! The software vendor of WEBI V3.1, BusinessObjects, does not support SQL Server 2012 at this time.
SQL Server 2012 is not officially supported with Sage ERP X3 prior to V6 patch 21.