Hello,
You have to use only one validation group for each validation control.
<asp:requiredfieldvalidator id="reqCode" runat="server" errormessage="*" controltovalidate="txtCode" validationgroup="valgrpCode" forecolor="Red" xmlns:asp="#unknown"></asp:requiredfieldvalidator>
<asp:requiredfieldvalidator id="reqDescription" runat="server" errormessage="*" controltovalidate="txtDescription" validationgroup="valgrpCode" forecolor="Red" xmlns:asp="#unknown"></asp:requiredfieldvalidator>
<asp:requiredfieldvalidator id="reqCategory" runat="server" errormessage="*" controltovalidate="txtCategory" validationgroup="valgrpCode" forecolor="Red" xmlns:asp="#unknown"></asp:requiredfieldvalidator>