add_filter( 'gform_validation', function ( $validation_result ) {

    $form = $validation_result['form'];

    // $forms_and_fields[child form id]
    $forms_and_fields[77] = [
        'name_field_id' => 5,
        'num_field_id'  => 3,
        'date_field_id' => 6,
    ];


    if ( in_array( $form['id'], array_keys( $forms_and_fields ) ) ) {

        $name_id = $forms_and_fields[ $form['id'] ]['name_field_id'];
        $num_id  = $forms_and_fields[ $form['id'] ]['num_field_id'];
        $date_id = $forms_and_fields[ $form['id'] ]['date_field_id'];

        $search_criteria = [];
        $search_criteria['status'] = 'active';

        $search_criteria['field_filters'][] = [
            'key' => $num_id . '',
            'value' => rgpost('input_' . $num_id),
        ];

        $search_criteria['field_filters'][] = [
            'key' => $name_id . '.3',
            'value' => rgpost('input_' . $name_id . '_3'),
        ];

        $search_criteria['field_filters'][] = [
            'key' => $name_id . '.6',
            'value' => rgpost('input_' . $name_id . '_6'),
        ];

        $search_criteria['field_filters'][] = [
            'key' => $date_id . '',
            'value' => date('Y-m-d',strtotime(rgpost('input_' . $date_id))),
        ];

        $entries = GFAPI::get_entries( $form['id'], $search_criteria );

        if ( count( $entries ) > 0 ) {
            $validation_result['is_valid'] = false;

            $fields = &$form['fields'];

            $field_ids = array_column( $fields, 'id' );

            $num_field_index = array_search( $num_id, $field_ids );
            $name_field_index = array_search( $name_id, $field_ids );
            $date_field_index = array_search( $date_id, $field_ids );

            $fields[ $num_field_index ]->failed_validation = true;
            //customize message
            $fields[ $num_field_index ]->validation_message = 'message';

            $fields[ $name_field_index ]->failed_validation = true;
            //custmize message
            $fields[ $name_field_index ]->validation_message = 'message';

            $fields[ $date_field_index ]->failed_validation = true;
            //custmize message
            $fields[ $date_field_index ]->validation_message = 'message';
        }
    }

    $validation_result['form'] = $form;
    return $validation_result;
} );