Hi All,

I have 3 DAG Node at one AD site, i Have 1 witness at CAS Sever

My question is, can Cluster still online if shuting down two mailbox?

Thanks

Yes cluster will be still online if your 2 nodes go offline.
In windows 2008 if your 2 nodes go offline automatically the 3rd node will be offline.
In windows 2012 has dynamic quorum enabled by default.
Please check my comments in the below threads.
https://social.technet.microsoft.com/Forums/office/en-US/26ff6190-5718-4b26-a557-c5a2a3009d49/exchange-2013-server-dr-planning?forum=exchangesvravailabilityandisasterrecovery

https://social.technet.microsoft.com/Forums/office/en-US/26ff6190-5718-4b26-a557-c5a2a3009d49/exchange-2013-server-dr-planning?forum=exchangesvravailabilityandisasterrecovery

Explained about dynamic quorum

http://www.msexchange.org/articles-tutorials/exchange-server-2013/high-availability-recovery/exchange-2013-dag-dynamic-quorum-part2.html

FYI in an odd number of DAG members File share witness is being ignored and "Node majority" in play.