Q:

Only display 3 tasks to a user at a time, then display 3 more once they have completed 3 tasks

I have 2 tables in my database.
First table is called tasks, which holds the task ID and the task. The second table is called completedTasks, which will hold the taskID and the username.
Once the user hits the 'Complete Task' button, it will post the taskID and their username to the completedTasks table. 
I am wanting to display 3 tasks to a user at a time, and once they have completed all 3 tasks, another 3 will appear. Currently, I am able to pull out 3 tasks from the database, but once the user clicks the 'Complete Task' button, it will pull out another task from the tasks table, to make it up to 3 again, instead of only displaying the further two that they need to complete.
Any suggestions on how I can display 3 tasks, then 2, then 1, then pull out another 3 would be great!
The code that I currently have is:
<?php
require ("dbConnect.php");
$user_check = $_SESSION['currentUser'];

$dbQuery = $db->prepare("select taskID, task from tasks WHERE taskID NOT IN (SELECT taskID from completedTasks WHERE username = '$user_check' ) LIMIT 3");
$dbQuery-> execute();

while ($dbRow = $dbQuery->fetch (PDO::FETCH_ASSOC)) {
    $taskID=$dbRow["taskID"];
    $task=$dbRow["task"];

    echo "<div align='center'>";
    echo "<form action='mytasks.php' method='post'>";
        echo "<input type='hidden' id='username' name='username' value='$user_check'>";
        echo "<input type='hidden' id='taskID' name='taskID' value='$taskID'>";
        echo "<table width='40%'>
                <tr>
                    <td><input type='text' id='task' name='task' disabled='true' value='$task'></td>
                    <td><input type='submit' name='submit' value='Complete Task'></td>
                </tr>
              </table>";
    echo "</form>";
    echo "</div>";
}

if (isset($_POST['submit'])){
    $username = $_POST['username'];
    $taskID = $_POST['taskID'];

    $dbQuery=$db->prepare("INSERT INTO `completedTasks` VALUES (:username, :taskID)"); 
    $dbParams = array('username'=>$username, 'taskID'=>$taskID);
    $dbQuery->execute($dbParams);
    if($dbQuery) {
        header('Location:mytasks.php');
        echo "<script>Success();</script>";
    }else{
        echo "<script>Unsuccessful();</script>";
    }
}

A:

$statement = $db->query("SELECT COUNT(*) from completedTasks WHERE username='$user_check' ");
$result = $statement->fetch();
$count = $result[0];
$count = $count-($count % 3);
$dbQuery2 = $db->prepare("SELECT taskID, task from tasks LIMIT 3 OFFSET $count");
$dbQuery2-> execute();

