#define pba 21
#define led 23
#define pbb 19
#define pbc 18

bool y;

void setup(){
  pinMode(led, OUTPUT);
  pinMode(pba, INPUT);  
  pinMode(pbb, INPUT);
  pinMode(pbc, INPUT);
}

void loop(){
  y=digitalRead(pba)&&digitalRead(pbb)&&digitalRead(pbc);
  digitalWrite(led,y);
}