Hi Skyarina

You should filter the rowsource of each combo box according to its parent in
the hierarchy.

For example, say your  rowsource for cboSubcategory is:
   Select SubCatID, SubCatName from SubCategories order by SubCatName;

You need to add a WHERE clause:
   Select SubCatID, SubCatName from SubCategories
       where CatID=Forms!YourForm!cboCategory
       order by SubCatName;

Also, you need to requery the "child" combo boxes in the AfterUpdate event
of the parent ones:

Private Sub cboCategory_AfterUpdate()
   cboSubCategory.Requery
   cboSubCategory = Null
   cboDetail.Requery
End Sub