I went back through my notes from the time I was working on the third UART...I was trying to enable the third port while keeping CAN and USB capabilities. I wasn't able to get that working because one of the pins - mapped to MFP09 - is used as an OE for a buffer on the SOM.
It's possible the third port is being mapped through the CANRX/TX multifunction pins.
Still trying to get my USB flash stick recognized.
John