Public Sub backblue(ByVal frm As Form, ByVal boxname As String)
      For i = 1 To 3
          CType(frm.Controls(boxname & i.ToString()), TextBox).BackColor = Color.LightBlue
      Next
   End Sub

Private Sub Button1_click and bla bla....
         backblue(Me, "txb1_")
      End Sub

Public Sub backblue(ByVal frm As Form, ByVal prefix As String)
   For i = 1 To 3
        Dim bxName as String = prefix & i.ToString()
        Dim bx as TextBox = CType(frm.Controls(bxName), TextBox)
       If bx Is Nothing Then
          MsgBox("Unable to find text box " +bxName)
          Dim mtch() As Control = frm.Controls.Find(bxName, true)
          If mtch.Length> 0 then
              bx = mtch(0)
          Else
              Continue For
          End if
        End If
       Bx.BackColor = Color.LightBlue
   Next
End Sub

For Each box as Textbox In frm.Controls
box.BackColor = Color.LightBlue
Next