@Damien7690 

The response is positive.  

  @yves_b bit new to this forum did the positive respone you sent mean this will work. Thanks in advance. Don't want to break anything and just try it as I have heard mixed results about when doing this it will slow the wifi right down on that third Deco unit

  @Damien7690 All devices are, obviously, competing to use the shared bandwidth.

Connecting the devices by cable to Deco is better, from that perspective, than connecting devices by wifi to Deco.

It is even much better if you can interconnect some Deco by cable, or connect more Deco to the router.

You do not provide any information about your usage and the available bandwidth, hence it is difficult to determine if the traffic of the five additional devices would have an impact on the usage of the existing devices.