[Bind(Include = "Test1,Test2,Test3")]
public  class TestModel2
    {
public string Test1 { get; set; }
public string Test2 { get; set; }
public string Test3 { get; set; }
public string Test4 { get; set; }
public string Test5 { get; set; }
    }

 [AcceptVerbs(HttpVerbs.Post)]
public ActionResult Test([Bind(Exclude = "Test1,Test2")]TestModel2 test)
        {
if (ModelState.IsValid)
            {
                test.Test3 = "123";
            }

return View();
        }