https://wokwi.com/projects/347478624767574611

Using 3 utrasound sensors without libraries and one LED

const byte numSensors = 3;
//const mean constant variables, byte means small space in memmory, numSensors is my variabe name.
const byte ECHO_PIN[numSensors] = {2, 5, 8};
const byte TRIG_PIN[numSensors] = {3, 4, 7};
*/ We create an ARRAY to be used in a for loop/*

void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
  for (byte s=0; s< numSensors; s++) {
  /*Due to foor loop we reduce pinMode instructions from 6 to only 2
   pinMode(3, OUTPUT);
   PinMode(2, INPUT);
   pinMode(4, OUTPUT);
   pinMode(5, INPUT);
   pinMode(7, OUTPUT);
   pinMode(8, INPUT);
  /*
    pinMode(TRIG_PIN[s], OUTPUT);
    pinMode(ECHO_PIN[s], INPUT);
  }
}

float readDistanceCM(byte s) {
/*This is a function wit a parameter (s) snd return a valuetaht dependson the duration of a pulseIn fucion and matehematical calculs with the velociy of sound*/
  digitalWrite(TRIG_PIN[s], LOW); 
  /* We need to have the trigger pin controled to a Low value in order to measure the duration ofthe high value that is proporcional to he distance of the sensor. */
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN[s], HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN[s], LOW);
  int duration = pulseIn(ECHO_PIN[s], HIGH);
  return duration * 0.034 / 2; 
  //Divided bay 2 because the ultrasound goes and returns back.
}

void loop() {
  bool isNearby = false;
  Serial.print("Measured distance: ");
  for (byte s=0; s < numSensors; s++) {
    float distance = readDistanceCM(s);
    isNearby |= distance < 100;
    Serial.print(distance);
    Serial.print(" ");
  }
  Serial.println();
  digitalWrite(LED_BUILTIN, isNearby);
  delay(100);
}
  
  delay(100);
}

https://www.tinkercad.com/things/1DIoZ7eNnYq-super-borwo/editel?sharecode=z4ZpapaceZIsQmAQ1eV_SMaoCBl1y3wKd38v4AneO8o

Using 3 utrasound (Work by Alex)

int ledPin1 = 2;
int ledPin2 = 3;
int ledPin3 = 4;

int trigPin1 = 6;
int echoPin1 = 7;

int trigPin2 = 8;
int echoPin2 = 9;

int trigPin3 = 10;
int echoPin3 = 11;

void setup() {
  Serial.begin (9600);
  pinMode(trigPin1, OUTPUT);
 // Trigger: el emissor del ultraasound.
  pinMode(echoPin1, INPUT);
 //Echo: reecbeix l'informació que emiteix el trigg sobre la posició de l'objecte que detecta el ultrasound.
 
  pinMode(trigPin2, OUTPUT);
  pinMode(echoPin2, INPUT);
 
  pinMode(trigPin3, OUTPUT);
  pinMode(echoPin3, INPUT);
 
  pinMode(ledPin1, OUTPUT);
  pinMode(ledPin2, OUTPUT);
  pinMode(ledPin3, OUTPUT);
  digitalWrite(ledPin3, HIGH);
}

void firstsensor(){ // This function is for first sensor.
  int duration1, distance1;
  digitalWrite (trigPin1, HIGH);
  delayMicroseconds (10);
  //Temps que tarda en arribar la inormació de trigg a echo. ()
  digitalWrite (trigPin1, LOW);
  duration1 = pulseIn (echoPin1, HIGH);
  distance1 = (duration1/2) / 29.1; //Velocidad del soniddo (29.1)

      Serial.print("1st Sensor: ");
      Serial.print(distance1); 
      Serial.print("cm    ");

  if (distance1 < 50) {  // Change the number for long or short distances.
    digitalWrite (ledPin1, HIGH);
  } else {
    digitalWrite (ledPin1, LOW);
  }   
}
void secondsensor(){ // This function is for second sensor.
    int duration2, distance2; 
    //I create two vaiables to olcal variable to hav an space in my microcontroler memory.
    
    digitalWrite (trigPin2, HIGH);
    
    delayMicroseconds (10); //Eimts ultrasound for 10 microsenconds.
    digitalWrite (trigPin2, LOW); //Stop eistion of ultrasoound
    duration2 = pulseIn (echoPin2, HIGH); //Activates echopin in high to reciebe a ultrasound backwards.
    //duration2 keeps the value obtinde by echopin2
    //It converte value of the ultrasound durations like a time of fly into a distance with using a mathematic formule.
    distance2 = (duration2/2) / 29.1;
      Serial.print("2nd Sensor: ");
      Serial.print(distance2); 
      Serial.print("cm    ");
  
    if (distance2 < 50) {  // Change the number for long or short distances.
      digitalWrite (ledPin2, HIGH);
    }
 else {
      digitalWrite (ledPin2, LOW);
    }   
}
void thirdsensor(){ // This function is for third sensor.
    int duration3, distance3;
    digitalWrite (trigPin3, HIGH);
    delayMicroseconds (10); //Temps que tarda en arribar la infromació de trigg a echo en microsegons.
    digitalWrite (trigPin3, LOW);
    duration3 = pulseIn (echoPin3, HIGH);
    distance3 = (duration3/2) / 29.1;

      Serial.print("3rd Sensor: ");  
      Serial.print(distance3); 
      Serial.print("cm    ");
  
    if (distance3 < 50) {  // Change the number for long or short distances.
      digitalWrite (ledPin3, HIGH);
    }
 else {
      digitalWrite (ledPin3, LOW);
      
      //"if"i "else": son entendidos por el propio ordenador mediate su chip y conecta la biblioteca a el codigo.
    }  
}

void loop() {
Serial.println("\n");
firstsensor();
secondsensor();
thirdsensor();
  delay(100);
}

#include 

Ultrasonic ultraleft(1, 2);  // (Trig PIN,Echo PIN)
Ultrasonic ultraright(3, 4); // (Trig PIN,Echo PIN)
Ultrasonic ultraup(5, 6); // (Trig PIN,Echo PIN)


int ledIzq = 7; // Pin del led izquierdo
int ledDer = 8; // Pin del led derecho
int ledArr = 9; // Pin del led arriba

void setup() {
  Serial.begin(9600);
}

void loop()
{
  int ledI;
  Serial.print("Left: ");
  Serial.print(ultraleft.read(CM)); // CM or INC
  Serial.print(" cm" );
  encenderLed(ultraleft.read(CM), 50, ledIzq);

ledI = map (distance,2,400,0,250);
analogwrite (7, ledI);


  delay(50);
  Serial.print("Right: ");
  Serial.print(ultraright.read(CM)); // CM or INC
  encenderLed(ultraright.read(CM), 50, ledDer);
  Serial.println(" cm" );

ledI = map (distance,2,400,0,250);
analogwrite (8, ledI);


  delay(50);
 Serial.print("Up: ");
  Serial.print(ultraup.read(CM)); // CM or INC
  encenderLed(ultraup.read(CM), 50, ledArr);
  Serial.println(" cm" );
  delay(50);

ledI = map (distance,2,400,0,250);
analogwrite (9, ledI);


}
void encenderLed(float cm, float umbral, int ledPin) {
  digitalWrite(ledPin, cm < umbral);
}

solution with LiquidCrystal Library Here