Setting Command Parameters For SQLDataSource

I have an SQLDataSource. The SQL is

 SELECT UserName, Category, ItemDescription, Size, Price, Reduce, Donate, Sold, ItemNumber, SoldDate, SoldPrice, Printed
FROM Tags
WHERE (Printed = @Printed1 OR Printed = @Printed2 OR Printed = @Printed3) ORDER BY ItemNumber DESC

The bit field "printed" can be NULL, True or False.

In the Selecting event of the SQLDataSource I have the following to show ALL records. But it does not work. If I remove these parameters it show ALL records.

e.Command.Parameters("@Printed1").Value = Nothing           'ASP.NET 2.0 using Visual basic
e.Command.Parameters("@Printed2").Value = True
e.Command.Parameters("@Printed3").Value = False


What am I doing wrong???

Thanks

Craig