This is a short guide restore the serial communication between MightyHat and your Pi3. You may also find this information in this official RaspberryPi.org UART documentation.
The new Bluetooth module on the Pi3 is configured to use the default /dev/ttyAMA0 (aka /dev/serial0) serial device on GPIO14-15. For compatibility with MightyHat, we need to disable the Bluetooth device and stop it from using that serial port. Edit the following file:
sudo nano /boot/config.txt
Add this line at the end:
Finally, make sure the config.txt file now has this line:
Save with Ctrl-X,Y. Then run this command:
sudo systemctl disable hciuart
Reboot your Pi3. Then install minicom, and if nothing else is using the /dev/ttyAMA0 serial port, you should be able to open it in minicom and see any incoming serial data from the MightyHat (ie RF data passed through to the Pi serial port):
sudo apt-get install minicom sudo minicom -b 19200 -D /dev/ttyAMA0
For reference and more details you may also want to read this helpful blog post that reviews the Pi3’s GPIO serial port and its usage with other hardware. There is also this blog post that discusses serial port issues on the RaspberryPi 3.