We've rebuilt v3 from scratch to leave a lot of legacy code behind. As a minimum requirement v3 will run on PHP 7.1+. This gave us so many great new ways of improving our code and make use of PHP 7's fantastic performance.
The panel is now based on Vue.js and our own REST API, which can be used for your own applications. This turns Kirby 3 into a very slick headless CMS out of the box.