The most effective way to implement nested grid layouts is by using jQuery Mobile's built-in grid system with clear block hierarchy. Here's the precise implementation:
Nested Grid Solution
<!-- Outer 2-column grid -->
<div class="ui-grid-a">
  <!-- First main column -->
  <div class="ui-block-a">
    <div class="ui-bar ui-bar-a">Main Column 1</div>   
    <!-- Nested 3-column grid inside first column -->
    <div class="ui-grid-b">
      <div class="ui-block-a"><div class="ui-bar">Nested A</div></div>
      <div class="ui-block-b"><div class="ui-bar">Nested B</div></div>
      <div class="ui-block-c"><div class="ui-bar">Nested C</div></div>
    </div>
  </div>
 
  <!-- Second main column -->
  <div class="ui-block-b">
    <div class="ui-bar ui-bar-a">Main Column 2</div>
    <!-- Nested 2-column grid inside second column -->
    <div class="ui-grid-a">
      <div class="ui-block-a"><div class="ui-bar">Nested X</div></div>
      <div class="ui-block-b"><div class="ui-bar">Nested Y</div></div>
    </div>
  </div>
</div>