Third device speed issues

I have 3x goCoax MoCA 2.5 adaptors configured as:

1 - Router -> goCoax

2 - goCoax -> WiFi node

3 - goCoax -> Apple TV

I find that when I have 2 devices connected they work as expected and operate a high speed (>1 Gbps). However as soon as I add a third device one of the connection speeds drop significantly (<10 Mbps). I have tried different aerial ports around the house and depending which aerial ports I use depends which line shows the slowest speed.

To illustrate if I connect just 1 and 2 above I'll get speeds of > 1Gbps. If I connect 1 and 3 I'll also get > 1Gbps. But if all 3 are connected then 1 and 2 will show 1Gbps but 1 and 3 will operate at < 10 Mbps. Hope that makes sense!

Is there a reason for this and a resolution?

Thanks