With Frontend Submissions installed and activated, the number of columns can be set programmatically for the single vendor page.

/**
 * Frontend Submissions: Set the number of columns for the single vendor page (Default is 2).
 *
 * @return int $columns The number of columns to display.
 */
function themedd_snippet_edd_fes_single_vendor_page_columns( $columns ) {

	$columns = 3;

	return $columns;
}
add_filter( 'themedd_edd_fes_single_vendor_page_columns', 'themedd_snippet_edd_fes_single_vendor_page_columns', 10, 1 );

PHP should be added to your child theme's functions.php file.