If you did so, the code would not be portable between different Arduinos, and error prone when editing in the case that the project wiring changes.
Try this and see if it does what you want:
void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
Serial.println(PB3);
digitalWrite(PB3, LOW);
}
void loop() {
// put your main code here, to run repeatedly:
}