Searching multiple tables in SQL
A quick way to resolve your issue would be simply to return a Null column in each of the select statements.

SELECT 
    url, NULL AS Category, NULL AS SubCategory,'Web' As Type FROM Website WHERE (url='www.facebook.com')
UNION ALL
SELECT 
     NULL, category, NULL AS SubCategory, 'Cat' As Type FROM Category WHERE (category='www.facebook.com')
UNION ALL
SELECT
    NULL, NULL, subCategory, 'Sub' As Type FROM SubCategory WHERE (subCategory='www.facebook.com')


This generate the following output which should match your requirement of output 

       url       | Category | SubCategory  | Type
www.facebook.com |  NULL    |    NULL      |  Web