I was able to connect 3 of these with no problems the other day, by adding them to a powerstrip. The trick is to connect the first 2 DHPs on a power line and then press the 'SECURE' button to secure the connection. Then you add the next one to the strip and repeat the 'SECURE' step.

For more information about using these FANTASTIC DHP powerline units, see my video below;
https://www.youtube.com/watch?v=sg8ljayEEsI
Ben