Hi,

Well, i see you have two options:

1) load the entire products table and do the filtering yourself using a
DataView:

private DataView categoryDV;
private DataView supplierDV;
private DataView productDV;
private Dataset ds = new Dataset();
// adapters...

private void Form1_Load(object sender, System.EventArgs e)
{
 // load all tables
 categoryAdapater.Fill(ds, "tbl_category");
 supplierAdapter.Fill(ds, "tbl_supplier");
 productAdapter.Fill(ds, "tbl_product");

 // create dataviews
 categoryDV = ds.Tables["tbl_category"].DefaultView;
 supplierDV = ds.Tables["tbl_supplier"].DefaultView;
 productDV = ds.Tables["tbl_product"].DefaultView;

 // bind
 catCb.DataSource = categoryDV;
 catCb.DisplayMember = "category";
 supDg.DataSource = supplierDV;
 prodDg.DataSource = productDV;

 UpdateProductFilter();

 // hook up category position changed (using CurrencyManager
 // returned from BindingContext).
 // warning: you must always use the same DataSource
 // (and DataMember) as the ones you used for binding
 BindingContext[categoryDV].PositionChanged+=
   new EventHandler(categoryCM_PositionChanged);

 // same for supplier
 BindingContext[supplierDV].PositionChanged+=
   new EventHandler(supplierCM_PositionChanged);

 // hook up listchanged to dedect new rows
 productDV.ListChanged+=
   new ListChangedEventHandler(productDV_ListChanged);
}

private void categoryCM_PositionChanged(object sender, EventArgs e)
{
 UpdateProductFilter();
}

private void supplierCM_PositionChanged(object sender, EventArgs e)
{
 UpdateProductFilter();
}

private void UpdateProductFilter()
{
 int catID = (int) // get current category_id
   ((DataRowView)BindingContext[categoryDV].Current)["ID"];

 int supID = (int) // get current supplier_id
   ((DataRowView)BindingContext[supplierDV].Current)["ID"];

 productDV.RowFilter =
    string.Format("(category_id = {0}) AND (supplier_id = {1})",
       catID, supID);
}

private void productDV_ListChanged(object sender, ListChangedEventArgs e)
{
 int catID = (int)
    ((DataRowView)BindingContext[categoryDV].Current)["ID"];

 int supID = (int)
    ((DataRowView)BindingContext[supplierDV].Current)["ID"];

 if ( (e.ListChangedType == ListChangedType.ItemAdded) &&
   (productDV[e.NewIndex].IsNew) )
 {
    // set the foreign keys
    productDV[e.NewIndex]["category_id"] = catID;
    productDV[e.NewIndex]["supplier_id"] = supID;
 }
}

OR (2) each time category or supplier changes you load only the revelant
rows using a DataAdapter with a parameterized select query.   This would
require a DataAdapter which has a select query with a parameter in it ,eg:
"SELECT * FROM tbl_products WHERE category_id = ? AND supplier_id = ?"

If you do this using the designer it will add the right Parameters to the
(Select)Command' Parameter collection also, if you do it from code, you need
to add the parameters youself:
see productDataAdapter.SelectCommand.Parameters.Add(...)

Once you have setup the right DataAdapter, you can then change the code
within UpdateProductFilter:

private void UpdateProductFilter()
{
 int catID = (int) // get current category_id
   ((DataRowView)BindingContext[categoryDV].Current)["ID"];

 int supID = (int) // get current supplier_id
   ((DataRowView)BindingContext[supplierDV].Current)["ID"];

 productAdapter.SelectCommand.Parameters("category_id").Value = catID;
 productAdapter.SelectCommand.Parameters("supplier_id").Value = supID;

 ds.Tables["tbl_product"].Clear();
 productAdapter.Fill(ds, "tbl_product");
}

Can't think of any now but parts can definitely be found on google, msdn or
codeproject....

HTH,
Greetings