Help me ,Serial1.begin on Arduino Mega 2560

I was uploaded this code on Arduino Mega 2560.

// Arduino Mega using all four of its Serial ports
// (Serial, Serial1, Serial2, Serial3),
// with different baud rates:

void setup(){

Serial.println("Hello Computer");
Serial1.println("Hello Serial 1");
Serial2.println("Hello Serial 2");
Serial3.println("Hello Serial 3");

void loop() {}

But Serial1.println ,Serial2.println and Serial3.println don't show Hello Serial.

This is result

Please help me. I am rookie.

Please use code tage when posting code; easier to read and copy.

Your can edit your post and add
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For the problem, only Serial communicates with the PC; you can not directly communicate with the PC using Serial1, 2 or 3. You will need e.g. a FTDI cable connected to the correct pins.