if ($_POST['n'] && is_iterable($_POST['n'])) {
	$columns = [];
	$values = [];
	$i = 1;
	
	foreach($_POST['n'] as $checked) {
		$columns[] = 'n' . $i;
		$values[] = intval($checked);
		$i++;
	}
	
	$query = 'INSERT INTO xxx (' . implode(', ', $columns) . ') VALUES (' . implode(',', array_fill(0, count($values), '?')) . ')';
	
	$stmt = $pdo->prepare($query);
	$stmt->execute($values);
}

<?php
if(isset($_POST['jouer'])){
	if(isset($_POST['n'])){
		$cases_cochees = [];
		foreach( $_POST['n'] as $valeur){
			$cases_cochees[] = $valeur;
		}
		
		echo "INSERT INTO xxxx (n1,n2,n3) VALUES ('{$cases_cochees[0]}','{$cases_cochees[1]}','{$cases_cochees[2]}');";
	}
}
?>

if( isset($_POST['jouer']) )
{
if(isset($_POST['n']))	
{
foreach( $_POST['n'] as $valeur )
   {
$n=$valeur;	
$sql = "INSERT INTO xxxx (n1,n2,n3) VALUES ('$n','$n','$n')";
$pdostat = $bdd->prepare($sql);
$pdostat->execute();
    }
}
}
?>
<form method='POST' action=''>
<center>
<br><b>COCHEZ 3 NUMÉROS</b>
<br><br>
<input type="checkbox" name="n[]" value="1" id="1">1<input type="checkbox" name="n[]" value="2" >2<input type="checkbox" name="n[]" value="3" >3
<input type="checkbox" name="n[]" value="4" >4<input type="checkbox" name="n[]" value="5" >5<input type="checkbox" name="n[]" value="6" >6
<br>
<input type="submit" name="jouer" value="jouer">
</form>