Well... it's not so easy as it seemed to be
It requires a little bit of hack...
Here we go
The problem you had is that you cannot access to the selecteditem in the itemdatabound event... I thought this was possible but it seems to not
So to fix this, first, add the OnSelectedIndexChanged handler to the ddl (this is the one who see the value of the selected item)
Now change  the onitemdatabound with the old one you had:

Public Sub eventItemDataBound1(ByVal S As Object, ByVal objArgs As DataGridItemEventArgs)

    If e.Item.ItemType = ListItemType.Item Or e.Item.ItemType = ListItemType.AlternatingItem  Then
        Dim drop1 As DropDownList = objArgs.Item.FindControl("myDDL")
        Dim myDataSet As DataSet   
        Dim myDataTable As DataTable

            myDataSet = CType(Session("data"), DataSet)
            myDataTable = myDataSet.Tables("Tabel")

        Dim myDataView As DataView

            myDataView = New DataView(myDataTable)
            drop1.DataSource = myDataView
            drop1.DataMember = "Tabel"
            drop1.DataTextField = "aantal"
            drop1.DataValueField = "aantal"
            drop1.DataBind()
               ' add this to initialize the third column  with the value 1 (or write it derectly on the aspx)
                e.Item.Cells[2].Text = 1;


    end  If
End Sub

And finally write this to the onselectedindexchanegd handler
Public Sub ddl_change(object sender, System.EventArgs e)
   Dim ddl as DropDownList = Ctype(sender, DropDownList) 
    ' get the datagrid  item being used bounded. Note that is the parent of the parent of the ddl
    Dim dgi As DataGridItem = Ctype(ddl.Parent.Parent, DataGridItem)
    dgi.Cells[2].Text = ddl.SelectedItem.Value
End sub

this is a hack but works... the problem is that teh databound event is being fired before the ddl change. If not you could store the selected item  in a public var and use it in the databound event