How to hide AJAX modal automatically ?

I have an ajax modal in page. I want it to disappear after e.g. 3 seconds without refreshing or post back on page. I tried TIMER but the postsback and looks weird. 

Any clean way to hide MODAL after few seconds.

<asp:ModalPopupExtender ID="mdlMessageBox" BackgroundCssClass="modalBackground" runat="server" TargetControlID="hdnField1"
PopupControlID="pnlMessageBox"></asp:ModalPopupExtender>