Séparateur tous les 3 chiffres à partir du 1er chiffre

=TEXTE(A1;REPT("###\ ";ENT(NBCAR(A1)/3))&REPT("#";NBCAR(A1)-3*ENT(NBCAR(A1)/3)))

=--SUBSTITUE(B1;" ";)

Private Sub Worksheet_Change(ByVal Target As Range)
Dim r As Range
Set r = Intersect(Target, [A:A], UsedRange)
If r Is Nothing Then Exit Sub
For Each r In r 'si entrées multiples
    If IsNumeric(CStr(r)) Then
        If Int(r) = r Then
            r.NumberFormat = Application.Rept("000\ ", Int(Len(r) / 3)) & String(Len(r) - 3 * Int(Len(r) / 3), "0")
        Else
            r.NumberFormat = "General"
        End If
    End If
Next
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Dim r As Range, t, n%, nn&
Set r = Intersect(Target, [A:A], UsedRange)
If r Is Nothing Then Exit Sub
If r.Count > 9999 Then t = Timer 'mesure de la durée à partir de 10000 cellules
Application.ScreenUpdating = False
r.NumberFormat = "General" 'RAZ
With Range(Replace(r.Address, ",", ":"))
    .Columns(2).Resize(, 2).Insert xlToRight  '2 colonnes auxiliaires
    With .Columns(2)
        .Cells(1, 2) = 1: .Columns(2).DataSeries 'repérage du classement
        .FormulaR1C1 = "=IF(ISNUMBER(RC[-1]),IF(NOT(MOD(RC[-1],1)),LEN(RC[-1])))"
        .Value = .Value 'supprime les formules
        .EntireRow.Sort .Columns(1) 'tri pour regrouper et accélérer
        For n = 4 To Application.Max(.Value)
            nn = Application.CountIf(.Cells, n)
            If nn Then
                Set r = .Cells(Application.Match(n, .Cells, 0), 0).Resize(nn)
                r.NumberFormat = Application.Rept("000\ ", Int(n / 3)) & String(n - 3 * Int(n / 3), "0")
            End If
        Next
        .EntireRow.Sort .Columns(2), xlAscending, Header:=xlNo 'ordre initial
        .Resize(, 2).Delete xlToLeft
    End With
End With
If t Then Application.ScreenUpdating = True: MsgBox "Durée " & Format(Timer - t, "0.00 \s")
End Sub