Licence key

• A minimum of 8 core licenses is required for each physical processor and a minimum of 16 core licenses is required for each server.
• Core licenses are sold in packs of two.
• Standard Edition provides rights for up to 2 Operating System Environments or Windows Servers containers with Hyper-V isolation when all physical cores in the server are licensed. For each additional 1 or 2 VMs, all the physical cores in the server must be licensed again. (regardless of the hypervisor used)
  
  

So in your specific case you'll either need a DataCenter edition license or with Standard edition you can also license the cores a second time to cover the additional (third) VM guest.

http://download.microsoft.com/download/7/2/9/7290EA05-DC56-4BED-9400-138C5701F174/WS2016LicensingDatasheet.pdf