Option Explicit
Private Sub Workbook_Open()
Dim lefichier As String
Dim mavar As Integer
Dim monfichier As String
Dim numfichier As String
mavar = 0
lefichier = Dir("DC-*.xls")
Do Until lefichier = ""
If Mid(lefichier, 4, 3) > mavar Then mavar = Mid(lefichier, 4, 3)
lefichier = Dir
Loop
Select Case Len(CStr(mavar))
Case 1: numfichier = "00" & mavar + 1
Case 2: numfichier = "0" & mavar + 1
Case 3: numfichier = mavar + 1
End Select
Range("A3").NumberFormat = "@" 'format texte
Range("A3") = numfichier
End Sub