Adding that code for the third UART from the Stormcore will work, just tested it. Will be nice to have another UART port on those.
Here is a compiled version of 5.3 with that change if you want
Unity_5.3_Firmware_3rd_UART_jaykup.bin (384.0 KB)
// Pins for Third UART
#define HW_UART_3_BAUD 115200
#define HW_UART_3_DEV SD2
#define HW_UART_3_GPIO_AF GPIO_AF_USART2
#define HW_UART_3_TX_PORT GPIOD
#define HW_UART_3_TX_PIN 6
#define HW_UART_3_RX_PORT GPIOD
#define HW_UART_3_RX_PIN 5
