title: dividing bootstrap grid into sub grid
body: <p>I have used bootstrap in my web application targeting all devices. I have divided the main row into three column(ex. second row ). Now I want to treat each divided column to another set of 12 columns each (like third row) and divide it. while dividing the main column to sub column, do I need to consider the screen size here ? If so, than what would be grid specification for it cause here generated column would be of different column width? or I have to use table tag for such purpose. </p>
  
  <p><img src="https://i.stack.imgur.com/5jzZJ.png" alt="enter image description here"></p>