Crystal Reports Problem with Select Expert and Multiple Parameters

Message

   I would like to have users select 1 of 3 parameters when the report runs.  I know how to create the parameters and use the Select Expert to lookup data on the report based on users input.  The problem is that the user should be able to choose from one of the three parameters.  However, two of them have a null value and when you select that from one of the parameter fields, the report gives you an error and says that this value is not valid.  I don't want them to force a value in there.  It should accept a null value and search by one of the other parameters listed.  Here is my Formula from the Select Expert.

{Command.stores_code} = {?Stores Code} or IsNull({?Stores Code}) or
{Command.stock_location} = {?Stock Location} or IsNull({?Stock Location}) or
{Command.Part_id} = {?Part Number}

So if Stores Code is null ..., then allow Stock Location or Part Number.  If Stock Location is null ..., then allow Stores Code or Part Number.  And if Part Number has a number in it, then allow Stores Code and Stock Location to be null.  This logic is not working.

Finally, if the users don't select any one of the three parameters, then it should list all records.  However, currently if they don't choose one of the three, then they get no records returned.  

I hadn't got that far yet, because I am working on the first half.  

Your help is greatly appreciated.