Sub test()
Dim Ws As Worksheet
For Each Ws In ActiveWorkbook.Worksheets
With Sheets("Dossiers Actifs")
If Not (.Range(.[A4], .Cells(Rows.Count, "A").End(xlUp)).Find(Ws.[C3]) Is Nothing) Then _
.Cells(Rows.Count, "A").End(xlUp)(2) = Ws.[C3]
End With
Next
End Sub