Markup:
                                <div class="row product-grid">
<?php $i = 0; foreach ($patisserieLarge as $key => $item) { $i++; ?>
                                        <div class="col-xs-12 col-sm-6 col-lg-4 product-item" data-product-key="<?php echo $key; ?>">
                                                <figure class="col-xs-6">
                                                        <a class="reveal product-thumbnail"><img alt="<?php echo $item['name'][$languageCode]; ?>" src="image/assortiment/<?php echo empty($item['images']['thumbnail-detail'])?'niemeijer-ontbrekend-beeld-assortiment-nl.jpg':$item['images']['thumbnail-detail']; ?>" class="img-responsive"><img alt="<?php echo $item['name'][$languageCode]; ?>" src="image/assortiment/<?php echo empty($item['images']['thumbnail'])?'niemeijer-ontbrekend-beeld-assortiment-nl.jpg':$item['images']['thumbnail']; ?>" class="img-responsive"></a>
                                                </figure>
                                                <div class="col-xs-6">
                                                        <p><a class="reveal"><?php echo $item['name'][$languageCode]; ?></a></p>
                                                        <p>&euro; <?php echo number_format($item['price'], 2, ',', '.'); ?><br><?php echo $item['short_description'][$languageCode]; ?></p>
                                                </div>
                                        </div><!-- product-key:<?php echo $key; ?> -->
<?php if ($i && !($i % 3)) { ?>
                                        <div class="clearfix visible-lg-block"></div>
<?php } ?>
<?php if ($i && !($i % 2)) { ?>
                                        <div class="clearfix visible-md-block"></div>
<?php } ?>
<?php } ?>
                                </div>

From the console:
$('.product-grid:eq(0) .product-item:eq(0)').index();
0
$('.product-grid:eq(0) .product-item:eq(1)').index();
1
$('.product-grid:eq(0) .product-item:eq(2)').index();
3
$('.product-grid:eq(0) .product-item:eq(3)').index();
5