This addon of fusion builder allows you to create a separate mobile layout for each of your page, post or portfolio element.
You can have a different layout of your page for mobile devices and a different layout for desktop and larger devices.
With avada you can have only 1 column per row in mobile layout, but this addon will allow you to have any number of columns per row in mobile layouts too.
Yes, you are right. You can show blog with 3 columns in desktop layout and with 2 columns in mobile layout of same page.
In settings of addon you can define the upper and lower limit of the mobile layout. Suppose you set the upper limit to 800px and lower limit to 360px. It means on all devices with width ranging from 360px to 800px you will see the mobile layout of your pages/posts/portfolio items. For devices with size greater than 800px or less than 360px will see normal avada view of page.