var all = new [] { btn1.ClientID, btn2.ClientID, btn3.ClientID };
var buttonId = Request.Form.AllKeys.SingleOrDefault(key => all.Count(item => key.Replace('$', '_').Contains(item)) > 0);

if (Request.Form.AllKeys.Length > 0)
{
    var buttonFakeId = Request.Form.AllKeys[Request.Form.AllKeys.Length - 1];
}

public  Control GetPostBackControl(Page page)
{
    Control control = null;

    string ctrlname = page.Request.Params.Get("__EVENTTARGET");
    if (ctrlname != null && ctrlname != string.Empty)
    {
        control = page.FindControl(ctrlname);
    }
    else
    {
        foreach (string ctl in page.Request.Form)
        {
            Control c = page.FindControl(ctl);
            if (c is System.Web.UI.WebControls.Button)
            {
                control = c;
                break;
            }
        }
    }
    return control;
}