PHP, MySQL Insert data from first table into second, third and fourth

Question

I have one table called users (id, username, password), second table called profiles (id, user_id, username, name, lastname, age, gender, country, company_name), third table called companies (id, user_id, name, resources_id) and fourth table called resources (id, user_id, company_id, money). When users register their account, they need to create their profile. Now when they fill profile data, profiles and companies tables are filled just fine.

My problem is in resources table:

Error: Incorrect integer value: '' for column 'company_id' at row 1

When I refresh that page with error, resources table is filled fine, but now I got double rows for profiles and companies tables.

My Code :

<?php
    session_start();

    $dbserver                    = "localhost";
    $dbusername                  = "root";
    $dbpassword                  = "1234512345";
    $db                          = "game";

    $conn = new mysqli($dbserver, $dbusername, $dbpassword, $db);

    if ($conn->connect_error)
    {
        die("Connection failed: ".$conn->connect_error);
    }

    if(isset($_SESSION['loggedin']))

    $username = $_SESSION['loggedin'];

    $query = "SELECT id FROM users WHERE username = '$username'";
    $result = mysqli_query($conn, $query);
    $row = mysqli_fetch_assoc($result);

    $userId = $row['id'];
    $_POST['id'] = $userId;

    $name = $_POST['name'];
    $lastname = $_POST['lastname'];
    $age = $_POST['age'];
    $gender = $_POST['gender'];
    $country = $_POST['country'];
    $company_name = $_POST['company_name'];
    $_POST['loggedin'] = $username;

    $query = "SELECT id FROM companies WHERE name = '$company_name'";
    $result = mysqli_query($conn, $query);
    $row = mysqli_fetch_assoc($result);

    $companyId = $row['id'];
    $_POST['id'] = $companyId;

    //INSERT DATA INTO PROFILES
    $sql = "INSERT INTO profiles (user_id, username, name, lastname, age, gender, country, company_name)
    VALUES ('$userId', '$username', '$name', '$lastname', '$age', '$gender', '$country', '$company_name')";

    //INSERT DATA INTO COMPANIES
    $sql2 = "INSERT INTO companies (user_id, name)
    VALUES ('$userId', '$company_name')";

    //INSERT DATA INTO RESOURCES
    $sql3 = "INSERT INTO resources (user_id, company_id)
    VALUES ('$userId', '$companyId')";

    if($conn->query($sql) && $conn->query($sql2) && $conn->query($sql3) === TRUE)
    {
        header("Location: ../../index.php?page=profile");
        die();
    }
    else
    {
        echo "Error: ".$conn->error;
    }
?>

What should I do differently? I'm still beginner into this, sorry for long post.

EDIT: One guy helped me out with this code, it's more error proof than last version. Deleted companies table, merged it with profiles, now everything works correctly!

<?php
session_start();

$dbserver                    = "localhost";
$dbusername                  = "root";
$dbpassword                  = "1234512345";
$db                          = "game";

$conn = new mysqli($dbserver, $dbusername, $dbpassword, $db);

if ($conn->connect_error)
{
    die("Connection failed: ".$conn->connect_error);
}

if(isset($_SESSION['loggedin']))

$username = $_SESSION['loggedin'];

$query = "SELECT id FROM users WHERE username = '$username'";
$result = mysqli_query($conn, $query);
$row = mysqli_fetch_assoc($result);

$userId = $row['id'];
$_POST['id'] = $userId;

$name = $_POST['name'];
$lastname = $_POST['lastname'];
$age = $_POST['age'];
$gender = $_POST['gender'];
$country = $_POST['country'];
$company_name = $_POST['company_name'];
$_POST['loggedin'] = $username;

$query = "SELECT id FROM companies WHERE name = '$company_name'";
$result = mysqli_query($conn, $query);
$row = mysqli_fetch_assoc($result); 

$sql1 = "INSERT INTO profiles (user_id, username, name, lastname, age, gender, country, company_name) VALUES (?,?,?,?,?,?,?,?)";
$sql2 = "INSERT INTO resources (user_id) VALUES (?)";
try {
  $conn->autocommit(false);
  $statement = $conn->prepare($sql1);
  $statement->bind_param("dsssdsss", $userId, $username, $name, $lastname, $age, $gender, $country, $company_name);
  $statement->execute();


  $statement = $conn->prepare($sql2);
  $statement->bind_param("d", $userId);
  $statement->execute();

  // COMMIT THE CHANGES
  $conn->commit();
  header("Location: ../../index.php?page=profile");
}
catch(Exception $e){
    // undo everything that was done in the try block in the case of a failure.
    $conn->rollback();
    echo "Error: ".$conn->error;
}?>