#include <LiquidCrystal.h> //include lcd library

#define BAUDRATE 9600
#define MAX_MESSAGE_LENGTH 64 //define max message length
String messageString; //define output String
String messageString1; //define output String

LiquidCrystal lcd(12, 11, 10, 9, 8, 7); //initialize lcd

void setup() {
  Serial.begin(BAUDRATE);
  Serial1.begin(BAUDRATE);
  Serial2.begin(BAUDRATE);
  Serial.println();
  lcd.print("TEST"); 
}

void loop() {
  if(Serial.available()){
     messageString = Serial.readStringUntil('\n');
     Serial1.print(messageString);
  }
  if(Serial2.available()){
    messageString1 = Serial2.readStringUntil('\n');
    lcd.clear();
    lcd.print(messageString);
    Serial.println(messageString1);
  }
}