const int red = 3;
const int ylw = 2;
const int grn = 1;
const int btn = 4;
int s;
int mode = 1;

void clear() {
  digitalWrite(red, 0);
  digitalWrite(ylw, 0);
  digitalWrite(grn, 0);
}

void setup() {
  pinMode(red, OUTPUT);
  pinMode(ylw, OUTPUT);
  pinMode(grn, OUTPUT);
  pinMode(btn, INPUT);
  clear();
}

void loop() {
  s = digitalRead(btn);

  if (s == 1) { 
    mode++;

    if (mode > 3) { mode = 1; }

    delay(500);
  }

  digitalWrite((mode-1), 0);
  digitalWrite(mode, 1);
}