$string = "kkjnmnmnjjjnmn";
$length = strlen($string);
$pieces = [];
for ($i = 0; $i < $length - 2; $i++) {
$piece = substr($string, $i, 3);
if (array_key_exists($piece, $pieces)) {
$pieces[$piece] += 1;
} else {
$pieces[$piece] = 1;
}
}
// $pieces will contain what you need