const byte SEG_PIN[3] = {15, 2, 16};   // 3-digit selection pins (common anode control)
byte Data[3] = {0, 0, 0};

const byte dataPin = 21;   // 74HC595 SER
const byte latchPin = 22;  // 74HC595 RCLK
const byte clockPin = 23;  // 74HC595 SRCLK

// 7-segment codes for numbers 0–9 (common anode, active LOW)
const byte LEDs[10] = {0xFC,0x60,0xDA,0xF2,0x66,0xB6,0xBE,0xE0,0xFE,0xF6};

// -------------------------------------------------------------------------------
void setup() {
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);

  for (byte i=0; i<3; i++) {
    pinMode(SEG_PIN[i], OUTPUT);
  }
}

// -------------------------------------------------------------------------------
void loop() {
  for (int i=0; i<1000; i++) {   // 000–999 loop
    show(i);
    delay(100);                 // 0.1 second per number
  }
}

// -------------------------------------------------------------------------------
void show(int number) {

  Data[0] = number % 10;        // Units
  Data[1] = (number / 10) % 10; // Tens
  Data[2] = (number / 100) % 10;// Hundreds

  for (byte i=0; i<3; i++) {

    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, LSBFIRST, LEDs[Data[i]]);
    digitalWrite(latchPin, HIGH);

    digitalWrite(SEG_PIN[i], LOW);  // Enable digit (common anode LOW)
    delay(5);                       // Multiplexing delay

    // Turn off all digits
    for (byte j=0; j<3; j++)
      digitalWrite(SEG_PIN[j], HIGH);
  }
}