MS Access Queries

I have a form in which I would like the values of the combination of three different combo boxes populate a subform based on a query. My issue is I can’t figure out how to get the queries to perform “on the fly”. The frist combo box (County) is filled in from a table. The second (City) is filled in as a query result from the same table and the value that the first Combo box gave me. The third (Address) is supposed to fill in based off of both the first and second values and the subform off of all three. The problem is, once the form is loaded, it has already generated the query based on the first value and never changes. How can I make this work?

Thanks.