Sub Макрос1() 
Dim UserRange As Range 
Dim UserAnswer As Integer 
Dim CountAdd As Integer 
Dim i As Integer

Do     
  Set UserRange = Application.InputBox("Выделите диапазон для добавления значений", "`Random 500`", Type:=8)     
  CountAdd = 0     
  Do         
    Do             
      i = Int(Rnd * 100)         
    Loop Until i <= UserRange.Count         
    If UserRange.Cells.Item(i).Value = "" Then             
      UserRange.Cells.Item(i).Value = "500"             
      CountAdd = CountAdd + 1         
    End If     
    Loop Until CountAdd = 3     
    UserAnswer = MsgBox("Обработать ещё один диапазон?", vbQuestion + vbYesNo, "`Random 500`") 
  Loop Until UserAnswer = vbNo 
End Sub