Hi, pi3 has changed the uart access a bit because of the onboard bluetooth. In short, you need to disable the onboard bluetooth and reassign the exposed uart gpio back to the PL011 which has better timing at different clockspeeds. It’s then accessed as before on /dev/ttyAMA0.
Add this to /boot/config.txt:
enable_uart=1
dtoverlay=pi3-disable-bt