What happens when Serial1 is directly connected to Serial2/3 on Mega 2560?

[SurferTim]

You are checking the wrong serial port for available. And use "while", not "if". Read it all, not just one character.

  while (Serial3.available()) {
    // get the new byte:
    char inChar = Serial3.read();
    Serial.print(inChar); // Send to PC what Serial3 received
  }

I seem to be a minute behind AWOL.