3 front sonar sensors for a autonomous car.

// Car finds middle path between right and left obstacles / walls / track boundaries ... MrLunk.net march-2017(gnu)
// Max track widt 2 meters.
// Max Right steer = 90+25 degrees servo angle. (max 115)
// Max Left steer = 90-25 degrees servo angle. (min 65)

#include <Servo.h>

// Define Sonar-Sensor Pinout numbers
const int trigPinOne = 5;
const int trigPinTwo = 6;
const int trigPinThree = 7;
const int echoPinOne = 8;
const int echoPinTwo = 9;
const int echoPinThree = 10;

Servo SteeringServo;

// define variables
long duration;
int Direction = 90;
int Distance;
int LeftDistance = 100;
int RightDistance = 100;
int MiddleDistance = 100;
int pos = 90;
int MinSideDistance = 100; // min Distance to side sensors before proportional steering kicks in.

void setup() {
  Serial.begin(9600);
  pinMode(trigPinOne, OUTPUT);
  pinMode(echoPinOne, INPUT);
  pinMode(trigPinTwo, OUTPUT);
  pinMode(echoPinTwo, INPUT);
  pinMode(trigPinThree, OUTPUT);
  pinMode(echoPinThree, INPUT);
  digitalWrite(trigPinOne, LOW);
  digitalWrite(trigPinTwo, LOW);
  digitalWrite(trigPinThree, LOW);
  SteeringServo.attach(11);
  delay (10);
  SteeringServo.write(90);
}

// --------------Main Routine definitions------------------------------

void loop() {

  // ---- Start Front Sensor Polling
  SonarSensor(trigPinOne, echoPinOne); // Poll Left Sonar-Sensor
  int LeftDistance = Distance;
  delay(10);
  SonarSensor(trigPinThree, echoPinThree); // Poll Right Sonar-Sensor
  int RightDistance = Distance;
  delay(10);
  SonarSensor(trigPinTwo, echoPinTwo); // Poll Middle Sonar-Sensor
  int MiddleDistance = Distance;
  delay(10);
  // ---- End Front Sensor Polling ----

  int LeftSteer = map(RightDistance,20,MinSideDistance,0,-25); // Proportional LeftSteer to Left object/wall distance
  if (LeftSteer <= -25){LeftSteer = -25;} // Max Right Sensor Error Protection

  int RightSteer = map(LeftDistance,20,MinSideDistance,0,25); // Proportional RightSteer to Left object/wall distance
  if (LeftSteer >= 25){LeftSteer = 25;} // Max Left Sensor Error Protection

  int Direction = (90+LeftSteer+RightSteer); // Final Direction calculation
  if (Direction <= 65){Direction = 65;}    // Max SteerLeft protection
  if (Direction >= 115){Direction = 115;}  // Max SteerRight Protection

  SteeringServo.write(Direction); // Send Final direction to Servo

  delay(50);
}

// --------------Sub-Routine definitions------------------------------

void Forward(){
}

void Reverse(){
}

void SonarSensor(int trigPin,int echoPin){
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  Distance = (duration/2) / 29.1;
  if (Distance > MinSideDistance) {
    Distance=MinSideDistance;
  }
}