@RobReid 

You didn't say what model the #1, #2, and #3 deco nodes were. Probably of no consequence, but...just in case. 

What Hardware version on the nodes and what firmware?

Have you tried resetting the Deco #3 back to factory settings (press the recessed button and hold for 10-15 seconds)?

That should initiate a flash back to factory settings. Then re-adding it back into the Mesh.  

If that does not work you may have run into a coincidental hardware failure, which has nothing to do with the new Node. In which case open a service ticket with Deco.

  @DAServices I tried a reset (didn't push for 10-15 seconds, but it did start flashing blue again). These units are all M4's and I just bought them this week. I will try the factory reset and see what happens...

  @DAServices I also tried the factory reset and it still doesn't work. I will put in a ticket.