is it possible that, for some reasons, the pi 3 tty is not enabled?
To verify try a "loop" connecting tty input with tty output to see if you receive the echo.
This is a very fasr test to see in there is a problem with the pi3 serial:
Please note that the s3 is a little bit different from the s2: the "good fast hw" serial is connected to bluetooth and the serial is a "poor man" sw one.
This sw UART is not capable to go as fast as 38400, so you be sure that you have switched the fase hw UART out of the BT.
Giovanni - HB9EIK