void loop() {
    for (int thisSensor = 0; thisSensor < 3; thisSensor++) {
       int sensorValue = analogRead(thisSensor);
       Serial.print(sensorValue, DEC);

       // if this is the last sensor value, end with a println().
       // otherwise, print a comma:
       if (thisSensor == 2) {
          Serial.println();
       } else {
          Serial.print(",");
       }
    }
 }