Option Explicit
Sub Essai()
Dim n&: n = Cells(Rows.Count, 1).End(3).Row: If n = 1 Then Exit Sub
Dim s$, k&, i&
Application.ScreenUpdating = 0: Columns(2).ClearContents: [B1] = "Réf 2"
For i = 2 To n
s = Cells(i, 1): k = Len(s)
If k > 2 Then Cells(i, 2) = Left$(s, k - 3) & StrReverse$(Right$(s, 3))
Next i
End Sub