Max, I do have one and I will try to do that. The problem is that the device is a slave that is listening to the serial port and responding - but I can make a spearate script just to test that.
I am pretty sure that I had previously an .ino script in Arduino IDE using stm32duino that worked at 2Mbps with the bluepill on the same UART3. I will dig out the bin try to flash it to see if that still works and will report back.
Thanks for the help.
Alex.