The textbox numbers you want to iterate should first be stored in an array. Then, using a for loop, iterate through that array and carry out your desired operation. Try the sub below.
Private Sub CommandButton1_Click()
Dim arr
Dim i As Long
arr = Array(3, 6, 8, 9)
For i = LBound(arr) To UBound(arr)
Me.Controls("TextBox" & arr(i)) = Me.Controls("TextBox" & arr(i)) * 2
Next i
End Sub