@rhbmcse 

This is a good idea to place additional unit in the middle. I would recommend going with same model of Deco, which is X50. In case you need to wall or ceiling mount new Deco unit, consider X50-PoE.

  @rhbmcse Assuming you are connecting to the two Decos wirelessly.  Does the second Deco have to be on the far side of the living room to get signal elsewhere in the house?  Have you tried moving it closer to the Deco in the entryway?  

  All sorted. 

I purchased an additional Deco and placed it between the two. 

Signal is now great between all decos. 

I just wondered how the decos would react by adding an additional unit.  I. E. Would they act independently in AP mode and therefore not increase the signal to the furthest away.

By adding the 3rd deco and telling them how to communicate upstream problem solved. 

Thanks for all of your help. 

Rob