Certifying 12.0-SP3 with Windows 2016 is not on the roadmap. It is certified with Windows 2012.
Here is a link to the current Platform Support Matrix for SiteMinder 12.0-SP3:
https://support.ca.com/phpdocs/7/5262/5262_12sp2_sp3_PSM.pdf
Here is a link to the SiteMinder Road Map:
https://support.ca.com/phpdocs/7/5262/5262_PlatformSupportRoadmap.pdf
As a side, service for most SiteMinder 12.0-SP3 components will end on July 15, 2016, though the web agents will continue to be supported with upgraded Policy Servers.
Here is the announcement:
CA SiteMinder r12.0 and CA SiteMinder Federation r12.0 End of Service Announcement
As Hubert suggested, you can post your request as an ideation.
Let me know if this answers your question.