HI RoAl_1242481 ,
I am not sure if I understood your query properly.
If you are looking for a third UART interface to connect a HW peripheral, then I am sorry its not possible with 20819.
There is only two HW UART interface possible - one is peripheral UART (PUART) and other one is HCI UART (which is used for programming the device. And later in the application can be used to send/receive raw packets as in MTB demo example under "HAL-20819EVB02- uart_raw_mode:).
Can you please confirm if the PUART is already connected with any other device? or please clarify your design requirement. Or please clarify the need of third PUART.
Thanks & Regards,