DRAG DROP
You administer three Microsoft SQL Server 2012 servers named ServerA, ServerB, and ServerC.
ServerA is the acting principal and ServerB is the mirror.
You need to add ServerC as a witness to the existing mirroring session between ServerA and ServerB.
You need to achieve this goal without delaying synchronization.
Which three actions should you perform in sequence? (To answer, move the appropriate actions
from the list of actions to the answer area and arrange them in the correct order.)
Answer: 
1 – 2 – 3
1 – 2 – 3
The steps are given here
https://msdn.microsoft.com/en-us/library/ms190430.aspx