Select at least one checkbox out of 3 required field validator

I am trying to make my code to put required field validator so at least one checkbox will be selected out of three.

This is my code:

<div class="left marginT5 marginR10">
 <input type="checkbox" id="chk1" value="option1" runat="server" class="styled" /> Choice1
 </div>
 <div class="left marginT5 marginR10">
<input type="checkbox" id="chk2" value="option2" runat="server" class="styled" /> Choice2
 </div>
 <div class="left marginT5 marginR10">
 <input type="checkbox" id="chk3" value="option3" runat="server" class="styled" /> Choice3
 </div>

I know I can use checkboxlist but because of design issues I can only use single checkbox controls. How can I "force" the user to select at least one checkbox?

You could try and use a Custom Validator. This should get you started:

aspx

<asp:CustomValidator ID="CustomValidator1" runat="server" 
    EnableClientScript="true"
    ErrorMessage="Please choose at least one"
    ClientValidationFunction="ValidateRadioButtons">
</asp:CustomValidator> 

javascript/jQuery

function ValidateRadioButtons(source, args) {
  var options = $('.styled');
  for (var i = 0; i < options.length; i++) {
    if (options[i].checked) {
      args.IsValid = true;
      return false;
    }
  }
  args.IsValid = false;
} 

Custom Validator

You should have server side validation too, just in case javascript is disabled.