I am currently using the board in question for the university project and I had a similar question to your's. We are using UART to get data from a GPS module. I've come to realize that while the chip has 3 UARTs, the board only provides connection to UART0 at the pins 3-RX and 1-TX. This is confirmed from the specification on the board at TinyTronics website (its a version 2 vs version 1, but still the same board).The interesting part here is that UART0 being the only external UART on the board, also acts as a programming channel for the board. This means that you can't use it for project purposes and for printing serial messages at the same time. Hope I was of help
Thank you for the information.I solved this problem when I discovered that quite every pin can be used for UART2 and UART3 in the "hardware serial" setup.
Is it realy true that I can use every pin for UART2/UART3? ...also with MicroPython?