Well - it really depends on what your table structures are, how they are related and what is the purpose of your queries.

If you want the results in a single resultset - then you are going to want to join or union. If you want three separate resultsets - then you will have three separate select statements.

We have no way of know what it is you are trying to accomplish without further information. Please review the article in my signature on how to post. It will help you post your question so we can answer it better.