[xmetal]

Не забываем про приведение типов
unsigned long result = (((unsigned long)data[0] << 16) + ((unsigned int)data[1] << 8) + data[2])/10;

byte data[3] = {17,38,174};

void setup() {
  Serial.begin(115200);
}

void loop() {
  unsigned long result = (((unsigned long)data[0] << 16) + ((unsigned int)data[1] << 8) + data[2])/10;
  Serial.println(result);
  delay(1000);
}