// category_l2 is the class name
// categories_l3 is a text line attribute name

$criterion = new CriterionLogicalAnd(
                array(
                    new CriterionSubtree(
                        $this->locationService->loadLocation( 68 )->pathString
                    ),
                    new CriterionContentTypeIdentifier(
                        'category_l2'
                    ),
                    new CriterionField(
                        'categories_l3',
                        CriterionOperator::CONTAINS,
                        'TEXT TO SEARCH'
                    )
                )
            );

$query->criterion = $criterion;
$content = $this->searchService->findSingle( $query->criterion );