Can the SQL Server 2012 service be running while applying CU 8 to SP 3?
You will definitely need to have users log off during the patch application; in theory, the service will be available for a while during the process, but there's no way to tell when the service will be stopped, and user activity could impact the patch application. 

Be safe, plan for a downtime, apply the patch. Make sure you have proper backups and recovery plans in place in case of problems.