The uart pins of the mcu are mapped to specific set of pins. So there are like 3 different sets of pins for each of uart drivers and you can select between them for each of the uarts. I accidentally selected two sets of uart 3 pins instead of one from uart 2 and one from uart 3. So you could use the ports alternating but not simultaneously. You can also use the pins for uart output at the side and i2c from the main but nobody really uses i2c except for sensors and such.