Third combo box optional

Hi,

Any advice on how to get a third combo box on a form to be optional.

I have a form with two combo boxes which filter out the results on a query.

However I want the third combo box to be optional i.e. if it is blank then just the results of the first two combo boxes are shown. If the user selects all combo boxes then obviously the results are based on all combo box selections.

As always any help appreciated.

Third combo box optional