<big><b>Gameplay</b></big>
Hearts of Iron 3 will allow the player to take control of almost any nation that existed or even plausibly could have existed during the period of 1936–1948, guiding it through World War II. A variety of decisions regarding the armed forces, production, research, diplomacy and politics is the focus of the game.


<big><b>Development</b></big>
The first trailer from the Games Convention in Leipzig showed new features such as 3D graphics.

Paradox released a series of developer diaries and video showcases.

Although happy with the scope of Hearts of Iron II, lead designer Johan Andersson wanted to improve upon every aspect of the game, starting again with a new engine. The game's artificial intelligence (AI) was designed to be able to achieve strategic objectives and control forces delegated to it, including whole theatres of operation. The AI can also remember and compare strategic possibilities as circumstances change. Changing to 3D graphics helped improve other areas, as Andersson explained:

"Going 3d meant we could do another type of architecture where we could support more screen resolutions, and develop our maps in a quicker way. The biggest advantage from going 3d though was the ability to offload more to the GPU. With the machine advances during the last decade this gives us the biggest benefits in development when it comes to gameplay and AI."

Paradox had a vision for the style of the map, given that the player would spend the majority of their time looking at it: "...to create a map that feels like a WW2 map, like it could be a map which...a commander in the War would be looking at himself." The content of the map was also altered; the number of provinces increased to more than 10,000 over Hearts of Iron II's 2,600. Customisable divisions, a first for the series, are available, each one containing one to five brigades, with each additional brigade increasing the division's combat power and cost.