#define DELAY_TIMER_BTN    B00000001                  // Button on A0
#define PROG_SEL_BTN       B00000010                  // Button on A1 
#define START_BTN          B00000100                  // Button on A2

unsigned long  prevmillis;                            // for debounce period
unsigned char  prevData;                              // previous reading of buttons
unsigned char  prevTog;                               // toggle memory
unsigned char  actSta;                                // buttons state
 
void setup() {                                        // pullups on pins A0,A1,A2  
  PORTC |= B00000111;                                 // A3, A4, A5 as output
  DDRC  |= B00111000;                                 //
  Serial.begin(115200);                               //
  Serial.println(F("go ! press a button"));           //
}                                                     //


void isButtonPressed() {
  if (millis() - prevmillis < 2) return;              // too soon to even look again (set to 20 IRL)

             prevmillis =  millis();                  // 20ms debounce delay reached
  unsigned char newData = ~PINC & B00000111;          // 0 when pressed, internal pullup on pins A0, A1, A2
  unsigned char prevSta =  actSta;                    // momentary save for toggle (see prevTog below)
  unsigned char   actOn =  newData & ~prevData;       // calculate PRESSED  transitioning bits
  unsigned char  actOff = ~newData &  prevData;       // calculate RELEASED transitioning bits, thanks alto777
                 actSta =  newData &  prevData;       // calculate STATE    of buttons
               prevTog ^= ~prevSta &  actSta;         // calculate TOGGLE   
               prevData =  newData;                   // bump along the history

  if (actOn & DELAY_TIMER_BTN) Serial.println(F("DELAY just pressed"));
  if (actOn & PROG_SEL_BTN)    Serial.println(F("PROG  just pressed"));
  if (actOn & START_BTN)       Serial.println(F("START just pressed"));
  
  if (actOff /* & DELAY_TIMER_BTN*/) Serial.println(F("released"));

  digitalWrite(A5, prevTog & START_BTN);              // toggle START/STOP red led  
  digitalWrite(A4, actSta);                           // when any button pressed, green led ON 
}

                                                      
void loop() {                                         // 1 to 16 BUTTONS,      reliable debounce,
  isButtonPressed();                                  // buttons info : state, fronts down & up, toggle,
}                                                     // multiple buttons can be pressed at a time