this is what i ended up doing: thanks for the help though Christian. I ended up changing them to dropdownlists but it works the same for textboxes.
Dim mystring As String = DropDownList3.SelectedItem.Text.ToString()
Dim substring As String = mystring.Substring(0, 5)
Dim mystring2 As String = DropDownList4.SelectedItem.Text.ToString()
Dim substring2 As String = mystring2.Substring(0, 4)
Dim mystring3 As String = TDropDownList.SelectedItem.Text.ToString()
Dim substring3 As String = mystring3.Substring(0, 3)
refnumber.Text = substring + "-" + substring2 + "-" + substring3