Hi,
Thank you for posting in Symantec community.
I would be glad to answer your question
Symantec has released SEP 12.1 RU3 (12.1.3001.165) in this month.
You can check SEP release details here: https://www-secure.symantec.com/connect/articles/what-are-symantec-endpoint-protection-sep-versions-released-officially
New fixes and features in Symantec Endpoint Protection 12.1.3
http://www.symantec.com/docs/TECH206828
You can directly upgrade from SEP 12.1 RU2 (12.1.2015.2015) to SEP 12.1 RU3 (12.1.3001.165).
If environment is big then can test the RU3 with few clients prior to roll out in the network.
Can refer this blog as well to understand more in depth about SEP 12.1 RU3
http://www.symantec.com/connect/blogs/whats-new-latest-symantec-endpoint-protection-sep-121ru3