char * p1 = "H";
char * p2 = "HH";
char * p3 = "HHH";

void printAll() {
  Serial.println(p1);
  Serial.println(p2);
  Serial.println(p3);
}

void setup() {
  Serial.begin(115200);
  printAll();
  *p1 = 'X';
  printAll();
}

void loop() {
  // put your main code here, to run repeatedly:

}