Ron

Open a "master" table in design mode.  Include the fields you wish to have a
combined key over.

Click on the <Index> button (or chose the menu item).  Enter a name for the
combined index in the first column.  In the second column, first row,
select/enter the first field.  Without adding a new name in the first
column, select/enter the second field in the second row of the second
column.

Now add a third field in the third row of the second column.

Check the bottom of this window to set the properties.