@dev1337  Hello, 

If  the main powerline unit (#1) could communicate with #2 and #3 and all 3 units shows up on the tpPLC Utility, that means they are in the same network and devices ( computers) connected to them could ping to each other, as all the devices obtain IP addresses from router.

Powerline rate between #2 to #3 or vice versa the is very slow and shows Red line, that means the power line environment is very poor or they are too far away from each other. 

 You could change a different wall socket for #2 and #3 and follow this instruction and remove the things that may affect the power environment:
https://www.tp-link.com/support/faq/409/