Couple of ways to do this, none of which are ideal, but try them and see what works in your environment. what you are currently doing is sending WHERE columnName in (NULL) when nothing is selected by the user. That's fine as long as you handle that in your logic. For instance if you had a stored procedure you were callign and passed a NULL value, you might handle that in the code with some IF syntax or something.

The best way for you would probably be this...Depending on how many values could be selected you could use of a multi select parameter and the SELECT ALL value. You could default it to SLEECT ALL by using the technique found here. That way when the report is run it automatically selects everything and returns a result set.

The only thing to watch out for is that if you default all 3 parameters to SELECT ALL when the report is run from the browser it will run automatically as soon as it's opened. Depending on the execution time and data you are bringing back this could be a problem as the user will have to wait for it to finish executing before they can narrow the results down.

Also if this is an expensive query from your db's perspective note how it may affect other users.

-Luke.