chunknshuffle

$shuffled = [];
$alphas = range('a', 'z'); // [a, ...z]
$pattern = [1, 2, 0];

foreach (array_chunk($alphas, 3) as $chk) {
    $r = [];
    foreach ($pattern as $i => $_) {
        $r[$i] = $chk[$pattern[$i]] ?? '';
    }
    array_push($shuffled, ...$r);
}

print_r(json_encode($shuffled));