You can check if the week changes inside the foreach as a possible approach.
<?php
$start = new DateTime( '2013-10-01' );
$end = new DateTime( '2014-01-01 23:59:59' );
$interval = new DateInterval('P1D');
$period = new DatePeriod($start, $interval, $end);
// only trigger every three weeks...
$weekInterval = 3;
// initialize fake week
$fakeWeek = 0;
$currentWeek = $start->format('W');
foreach ($period as $date) {
if ($date->format('W') !== $currentWeek) {
$currentWeek = $date->format('W');
$fakeWeek++;
print ' WEEK ' . $currentWeek . '<br/>';
}
if ($fakeWeek % $weekInterval !== 0) {
continue;
}
$dayOfWeek = $date->format('l');
if ($dayOfWeek == 'Monday' || $dayOfWeek == 'Wednesday' || $dayOfWeek == 'Friday') {
print $date->format('Y-m-d H:i:s') . ' ' . $dayOfWeek . '<br/>';
}
}