test2.php

<?php
$arr = [
    ['R1', 'R2'],
    ['A1', 'A2', 'A3'],
    ['M1', 'M2']
];
for ($i = 0; $i < count($arr[0]); $i++){
    for ($j = 0; $j < count($arr[1]); $j++) {
        for ($k = 0; $k < count($arr[2]); $k++) {
            echo $arr[0][$i] . $arr[1][$j] . $arr[2][$k];
            echo '<br>';
        }
    }
}