// limitations:
// no debounce, but this should not be a problem
// in case of very short press, risk of confusion on the buttons (maybe it will be almost never seen) 
// only one button pressed at a time


#define DELAY_TIMER_BTN    B00000001                  // PCF pin 0  (write  B00000001 or 1)
#define PROG_SEL_BTN       B00000010                  // PCF pin 1  (write  B00000010 or 2)
#define START_BTN          B00000100                  // PCF pin 2  (write  B00000100 or 4)

unsigned long  prevmillis;                            // tempo
byte           dataFromPCF;                           // PCF result



void setup() {
  PORTC |= B00000111;   
  Serial.begin(115200); 
  Serial.println(F("go ! press a button"));  
}


byte readDataFromPCF() {
  byte lowerByte = ~PINC & B00000111;
  return lowerByte;                                   // PCF return positive logic (1 when pressed, pulldown on pin)
}



void function1(){                                     // here your function when DELAY_TIMER_BTN is pressed
  Serial.println(F("DELAY_TIMER_BTN just pressed"));  //
}                                                     //
void function2(){                                     // here your function when PROG_SEL_BTN is pressed
  Serial.println(F("PROG_SEL_BTN just pressed"));     //
}                                                     //
void function3(){                                     // here your function when START_BTN is pressed
  Serial.println(F("START_BTN just pressed"));        //
}                                                     //



void isButtonPressed() {
  if (millis() - prevmillis > 1) {                    // wait for 50 ms period
    prevmillis = millis();                            // 50ms reached

    dataFromPCF <<= 1;                                // left shift
    dataFromPCF += readDataFromPCF();                 // add button status

    if (dataFromPCF == DELAY_TIMER_BTN) function1();  // 
    if (dataFromPCF == PROG_SEL_BTN)    function2();  //
    if (dataFromPCF == START_BTN)       function3();  //
  }
}



void loop() {
  isButtonPressed();
  //...
  
}