Hello, you recently purchased AV600 KIT, it is recommend to exchange it to AV1000 model to get the best performance. Please check 2.There are TP-Link powerline devices in the house already for more details:
To add a third unit, locate the third unit closer to one of the old extenders, and manually pair them by pair button referring to the instruction:
Note: press Pair button only for 1 sec, Power Leds should be flashing on both powerline units in pairing process. ( if the Power Led does not blink, press it for 1 sec again) When the Powerline Led or House Leds are on, the pairing process is done.
Or refer to this instruction to pair the powerline units/ add an extra powerline unit by Powerline Key:
How to add a device using the tpPLC App (Smartphone) https://www.tp-link.com/support/faq/1690/