#include "ssd.h"

const unsigned char digits[] = {ZERO, ONE, TWO, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE};
static unsigned char ssd[MAX_SSD_CNT];

void setup() {
  init_ssd();
}

void loop() {
  static int count;
  static int wait = 0;

  ssd[0] = digits[(count / 100) % 10];
    
  ssd[1] = digits[(count / 10) % 10];

  ssd[2] = digits[(count / 1) % 10];
 
 

	display(ssd);

  if (!wait--) {
    wait = 5000;
    count++;

    if (count ==  999)
      count = 0;
  }
  
}

sketch.ino

diagram.json

#include <Arduino.h>
#include "ssd.h"

void init_ssd(void) {
	int pin;

	/* 
	* NOTE: The below loop intializations depends on the 
	* selected pins configuration
	*/

	/* 
	* Initializing data lines 
	* Pin No   : 0  1  2  3  4  5  6  7
	* Segment  : E  D  C  H  G  F  A  B
	* Find the font definition in the header file
	*/
	for (pin = 0; pin < MAX_DATA_PINS; pin++)
		pinMode(pin, OUTPUT);

	/* 
	* Initializing control lines
	* Pin No   :  8  9  10
	* Display  :  D1 D2 D3
	*/
	for (pin = PIN_BASE; pin < (PIN_BASE + MAX_SSD_CNT); pin++)
		pinMode(pin, OUTPUT);
}

/* Better if implemented in timer handler */
void display(unsigned char data[]) {
	unsigned char digit;
	int pin;

	for (digit = 0; digit < MAX_SSD_CNT; digit++) {
		/* Make sure all the display is OFF to start with */  
		for (pin = 0; pin < MAX_SSD_CNT; pin++)
			digitalWrite(PIN_BASE + pin, OFF);

		/* Put the data on the pins */
		for (pin = 0; pin < MAX_DATA_PINS; pin++)
			digitalWrite(pin, !!((data[digit] << pin) & 0x80));
		
		// 0x6B - 01101011
		//        (01101011 << 0) & 10000000) -> 00000000 -> !!00000000 -> 0
		//        (11010110 << 1) & 10000000) -> 10000000 -> !!10000000 -> 1
		//        (10101100 << 2) & 10000000) -> 10000000 -> !!10000000 -> 1
		//        (01011000 << 3) & 10000000) -> 00000000 -> !!10000000 -> 0
		//        (10110000 << 4) & 10000000) -> 10000000 -> !!10000000 -> 1
		//        (01100000 << 5) & 00000000) -> 00000000 -> !!10000000 -> 0
		//        (11000000 << 6) & 10000000) -> 10000000 -> !!10000000 -> 1
		//        (10000000 << 7) & 10000000) -> 00000000 -> !!10000000 -> 1

		/* Enable a specific display */
		/*
		 * Pin No:  8  9  10
		 *          0  1  1
		 *          1  0  1
		 *          1  1  0
		 */
		for (pin = 0; pin < MAX_SSD_CNT; pin++)
			digitalWrite(PIN_BASE + pin, !(pin == digit));
			// pin == digit
			// First iteration
			// 0   == 0 -> TRUE -> 1 -> !1 -> 0
			// 1   == 0 -> FLASE-> 0 -> !0 -> 1
			// 2   == 0 -> FLASE-> 0 -> !0 -> 1
			// Second iteration
			// 0   == 1 -> FALSE-> 0 -> !0 -> 1
			// 1   == 1 -> TRUE -> 1 -> !1 -> 0
			// 2   == 1 -> FLASE-> 0 -> !0 -> 1
			// Third iteration
			// 0   == 2 -> FALSE-> 0 -> !0 -> 1
			// 1   == 2 -> FLASE-> 0 -> !0 -> 1
			// 2   == 2 -> TRUE -> 1 -> !1 -> 0
	}
}

ssd.cpp

#ifndef SSD_H  
#define SSD_H  

#define MAX_SSD_CNT 3
#define MAX_DATA_PINS 8

#define PIN_BASE 8

#define OFF 1
#define ONE 0

/* 
  * Font macro creations for Common Cathode (CC) display
  * Pin No   : 0  1  2  3  4  5  6  7
  * Segment  : E  D  C  H  G  F  A  B
  * ZERO     : 1  1  1  0  0  1  1  1  -> 0xE7
  * ONE      : 0  0  1  0  0  0  0  1  -> 0x21
  * Find the other definitions below   
  */
#define ZERO				    0xE7
#define ONE					    0x21
#define TWO					    0xCB
#define THREE				    0x6B
#define FOUR				    0x2D
#define FIVE				    0x6E
#define SIX					    0xEE
#define SEVEN				    0x23
#define EIGHT				    0xEF
#define NINE				    0x6F
#define DOT					    0x10
#define BLANK				    0x00
/* Need to fill all others */
#define M_ONE				    0x9D
#define MINUS				    0xFD

void init_ssd(void);
void display(unsigned char data[]);

#endif