Thought I knew where one is, but I can't find it.
Actually, it is not that hard.
Lets say we want to filter by Company and Department.
Lets set up two combos.  cboComp and cboDept
We only want to see departments in cboDept for the company selected in
cboComp.

So we set up our row source for cboComp
  SELECT CompID, CompName FROM tblCompany;

Where The bound column is 1, which will return CompID for the selected row.

Now we set up the row source for cboDept to filter on the company
   SELECT DeptID, DeptName FROM tblDept WHERE CompDept = Me.cboComp;

Where CompDept is the field in tblDept that is the foreign key to tblCompany

Now in the After Update event of cboComp

   Me.cboDept.Requery

Simple as that.  When you select a company, only that company's departments
will be listed in cboDept.

So, one step further, we only want to see Contacts in the Department
selected.  We need a row source for cboContact that filters on the selected
department

   SELECT ContactID, ContactName FROM tblContact WHERE DeptContact =
Me.cboDept;

And again, in the After Update of cboDept

   Me.cboContact.Requery