Hi,
I suppose that you use FRDM-MCXN947 board, from the hardware perspective, the UART4 of MCXN947 is used to communicate between the MCXN947 and debug probe chip LPC55S69.

The P1_9/FC4_P1_UART_TXD_MCULINK of MCXN947 is connected to Uart_RX of LPC55S69, P1_8/FC4_P0_UART_RXD_MCULINK of MCXN947 is connected to UART_TX of LPC55S69.
When you connect the MCU-LINK port of FRDM-MCXN947 board to PC USB, the UART of LPC55S69 is enumerated as a virtual COM port, so you can communicate with the uart 4 of FRDM-MCXN947 board and PC.
If you want to use UART3 of FRDM-MCXN947 board, I suppose it is okay.
P1_12 is FC3_P0, connected to pin28 of J9
P1_13 is FC3_P1, connected to pin27 of J9
You can use a SCI-USB board, connect the P1_12/P1_13/GND to the SCI-USB board.
From software,
You have to change the
#define #BOARD_DEBUG_UART_INSTANC 3
DbgConsole_Init(BOARD_DEBUG_UART_INSTANCE, BOARD_DEBUG_UART_BAUDRATE, BOARD_DEBUG_UART_TYPE, uartClkSrcFreq);
You have to change the pin_mux.c to assign the P1_12/13 to FC3 function.
Hope it can help you
BR
XiangJun Rong