Apply SP3 on SQL Server 2012 AlwaysOn AAGs
Before an upgrade, you should:


Perform a full backup of every database
Try a manual failover. Make sure the AG fails over a Synchronous replicas and that the synchronization state of the replicas is SYNCHRONIZED
Disable automatic backups on Secondary while they are being upgraded
Disable automatic backups on Primary before the upgrad of Primary
Do not upgrade Primary before any or all of the Secondary or .


Then here is the Rolling Upgrade Process:


Remove automatic failover on all synchronous-commit replicas
Upgrade all remote secondary replica instances running asynchronous-commit secondary replicas
Upgrade the all local replica secondary instances that are not currently running the primary replica
Manually fail over the availability group to a local synchronous-commit secondary replica
Upgrade or update the local replica instance that formerly hosted the primary replica
Configure automatic failover partners as desired