<?php

$names = Array
(
0 => Array('idn' => '8','name' => 'Петя',),
1 => Array('idn' => '7','name' => 'Ира',),
2 => Array('idn' => '3','name' => 'Вася',),
3 => Array('idn' => '5','name' => 'Дима',),
4 => Array('idn' => '6','name' => 'Семен',),
5 => Array('idn' => '1','name' => 'Влад',),
6 => Array('idn' => '2','name' => 'Глеб',),
7 => Array('idn' => '4','name' => 'Юля',),
8 => Array('idn' => '9','name' => 'Таня',)
);

$count = count($names);

if ($count > 0) {
    echo "<ul>\n";
}

for ($i = 0; $i < $count; $i++) {
    echo '<li>' . $names[$i]['name'] . ':' . $names[$i]['idn'] . "</li>\n";
    if (((($i+1) % 3) == 0) || $i+1 == $count) {
        echo "</ul>\n";
        if ($i+1 !== $count) {
            echo "<ul>\n";
        }
    }

}