Can you upgrade a single SQL Server 2008 Failover Cluster Instance to SQL Server 2012?
This is possible and we successfully completed an upgrade of a single failover cluster instance to SQL 2012 on a cluster with multiple SQL 2008 failover cluster instances.

Perform the upgrade of the instance on the two nodes not hosting the instance first (be sure to select the correct instance when asked as in the screenshot), then finally perform the upgrade on the node hosting the instance.