test3.php

<?php
$arr = [
    ['R1', 'R2', 'R3'],
    ['A1', 'A2', 'A3'],
    ['M1', 'M2']
];

$result = [""];
foreach ($arr as $item) {
    $tmp = [];
    foreach ($item as $el) {
        foreach ($result as $curr) {
            array_push($tmp, $curr . $el);
        }
    }
    $result = $tmp;
}
echo json_encode($result);