If the first two are already stacked, there is no need to power them down to add the third unit. Ensure the third unit is on the same firmware, connect the stacking cables to the additional switch, and then power it on. During bootup it will detect the stack and become a member. If none of the switches are stacked, then you will need to power them down to initially setup the stack. Here is a KB article to help with that.  [URL] 
Hi All, Actually I have bought 2 N2000 series and now I have bought another one. (So I have 3 now).  I have the stacking cables. My Question is : - how do I stacking the 2 switch with 1 new switch? (Best Practice) - Do I need to shutdown first the 2 switch to deploying stack cable?- Is there any document to troubleshoot if I fail doing stack?  Please kindly need your help.  Thanks before,
Stacking N2000 Series