modal box hiding issue in asp.net

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Hii Friend
in my applications i implemented gridview export to ppt before exporting it's ask confirmbox if click "yes" it's export otherwise it's can't
here my issue is i am using bootstrap modal box after exporting that modal box is not hiding

iam using this code in .aspx

<div class="modal hide fade" id="Div3">
                        <div class="modal-header">
                            <button type="button" id="Button31" class="close" data-dismiss="modal" aria-hidden="true"
                                 runat="server">
                                ×</button>
                            <h3>
                                </h3>
                        </div>
                        <div class="modal-body">
                           
                        </div>
                        <div class="modal-footer">
                            
                                  <asp:Button ID="Button3" runat="server" Text="Yes" class="btn btn-primary" 
                                        autopostback="true" onclick="Button3_Click" CausesValidation="False" />
                                    <asp:Button ID="BtnCancel" runat="server" Text="No" class="btn btn-primary" 
                                        autopostback="true" onclick="BtnCancel_Click" />
                               
                        </div>
                    </div>

in code behind i wrote this code

ScriptManager.RegisterStartupScript(this, GetType(), "modalscript", "$('#Div3').modal('hide');", true);

here my issue is
after downloading complete the modal box is not hidig