Open up the Data Assist search server behavior on the results page. Remove the SKU and Long Description comparisons. Then edit the Product Name one... Multi-select the three fields using control+click when editing the product name comparison and bind it to the <?php echo(isset($_GET['ProductName'])?$_GET['ProductName']:""); ?> value.
When you apply the search it should only have one row in the search fields, but that field should search across the three columns.