modification tableau

Private Sub CommandButton1_Click()
Dim i&, J&, L&
Dim T As Variant, Treport As Variant

With Sheets("Tableau initial") ' A Adapter
    T = .Range(.Cells(1, 1), .Cells(.Rows.Count, 1).End(3)(1, 7))
End With

ReDim Treport(1 To UBound(T, 1) * UBound(T, 2), 1 To 3)

For i = LBound(T, 1) + 1 To UBound(T, 1)
    For J = LBound(T, 2) + 1 To UBound(T, 2)
        L = L + 1
        Treport(L, 1) = T(i, 1)
        Treport(L, 2) = T(1, J)
        Treport(L, 3) = T(i, J)
    Next J
Next i

With Sheets("Importation") 'A Adpter
    .UsedRange.ClearContents
    .Cells(2, 1).Resize(L, UBound(Treport, 2)) = Treport
End With
    
End Sub

Option Explicit

Private Sub Worksheet_Activate()
Dim Te(), Ts(), Le&, Ce&, Ls&
Te = Feuil1.UsedRange.Value
ReDim Ts(1 To (UBound(Te, 1) - 1) * (UBound(Te, 2) - 1), 1 To 3)
For Le = 2 To UBound(Te, 1)
   For Ce = 2 To UBound(Te, 2)
      Ls = Ls + 1
      Ts(Ls, 1) = Te(Le, 1)
      Ts(Ls, 2) = Te(1, Ce)
      Ts(Ls, 3) = Te(Le, Ce)
      Next Ce, Le
Me.UsedRange.ClearContents ' (Ajouté suite à lecture du code de Efgé)
Me.[A2].Resize(Ls, 3).Value = Ts
End Sub