I was a big fan of HOI2 and played it a lot. It had a good balance of playability and realism and there were some issues which needed fixing, such as convoys and the occasionally flaky AI but it was a good game.
HOI3, sadly, is not. It has taken me a while to work out why, but I think it's because, in trying to make it a complete war simulation, the developers have failed to either make it accurate or make it fun. The accuracy claim sounds harsh, so let me explain.
1 - Research. There's no logic to it. One has to seperately research the gun carriages and weapons for anti-tank guns, for anti-aircraft guns, and artillery. That's six separate thingies to research. Then there's a seventh bit for heavy AA for your provinces.
Now in reality, what happened was that the best anti-tank weapons (British 17pdr, German 88, American 3") all started life as AA guns. It would make far more sense to have a 'foundry' option where guns could be researched and then applied to different areas.
Why even force people to research three lots of gun carriages? It makes absolutely no sense.
The same applies to separate research for cavalry weapons, militia weapons, and infantry weapons. What, we're all using different rifles now?
As a minor aside let me come on to the logic behind the naming of each progressive unit. For example, the British medium tank starts as the A9 cruiser, then when you research 1940 tanks it becomes the A15 Crusader (skipping the A13), then for 1942 tanks it becomes the Sherman Firefly. What? No Cromwell? Why the Firefly? This isn't the only case of a total lack of logic in the choice of naming. The naming doesn't affect the game but for a game which boasts of its realistic nature it is totally out of place.
IMO the complexity of the research tree does NOT add anything to the game. I think it could have been halved in size and been far more enjoyable to use without losing any gameplay.
2 - Convoys. The problem with convoys is that there is no mechanism to manage their routes. This was a problem in HOI2 where the German AI could sink the entire British convoy fleet by hanging around in a few areas of the North Sea and continually sinking ships. The very least HOI3 should have had is a convoy management system. The player should be able to tell the convoy routing system NOT TO SAIL through certain areas. Instead I have to watch convoys being sunk in the middle of the North Sea and around the English Channel, instead of rerouting all my convoys to the NW coast of Scotland or Liverpool and Belfast (as was done historically). It's a huge oversight and it is one of the primary problems with playing as the British. One of the other problems is that the cost of both convoys and escorts is fixed and does not fall over time (as it did historically with Liberty Ships and corvettes). It's almost as though the game is written solely with land combat in mind and about 3/4 of the way through someone said "Hey, wasn't there someone else in WW2 beside Germany and Russia?"
It's a real problem and affects playability and the prospect of a historical outcome to WW2.
3. Naval AI. While I'm on the subject, the AI doesn't seem to know what to do with its ships. The Germans send their battleships out unescorted to hang around the west coast of England and get sunk. Which they do. A lot. It makes me suspect that the major emphasis for the game was on land combat and naval was not addressed properly.
4. Allies. In HOI2 getting allies to fight for or with you was easy. They often sent you units, and there was an option for you to take over their entire armed forces. In HOI3 that option doesn't exist. And they don't send you units. You have to 'set objectives' to get the allies to work with you. These objectives don't really work. So, when playing as Britain, Canada keeps its air force and navy and army in Canada. When you urgently need Canadian fighters to defend your airspace and Canadian destroyers to patrol the Atlantic, they won't budge. There is no way to unify command, so you don't fight as allies but merely as participants in a global war where you aren't fighting each other. It's poor.
5. Crashes. Oh my goodness, the game's unreliability is terrible. It crashes to desktop the whole time. To minimise the inconvenience of losing my game progress I set it to autosave every game week, which is the most frequent autosave setting. However, if you are in the middle of a huge amount of preparation for your forces (reorganising your divisions for example) the game might crash. This has happened a couple of times to me, where I've been playing as Soviets and have been engaged in a major reorganisation of my 300+ divisions, and the game has just crashed to the desktop, and of course the autosave hasn't kicked in because I'm not progressing time through the game. HOI2 never did this, and it's by far the most infuriating feature of the game.
6. Graphics and Interface. It's cluttered and it's illogical and it's poorly laid out and it's not designed to work on higher resolution settings. It's a bloody mess. And let's talk about the little pictures for each unit that appears in the research window and the production window. Apparently it was too much effort for the devs to dig out the pictures of the units that they used in HOI2. As a result far too many units, which had the correct photo in the previous game, have a 'stock' photo in this game.
There are historical oddities, too, ones which ought to have stood out like a sore thumb. It is now possible to build (assuming you have the IC) lots of provincial AA without expending any manpower. Who's manning all those guns? Historically Nazi Germany had to commit a million men to defending their homeland from bombers, and that was a drain on manpower. In HOI3, they don't have to. They had to in HOI2.
I could live with the game if it was new. If it had been out for a couple of months I'd say "Well, every game ships with bugs so I can live with it for now." But this isn't a new game, it's been out for years, and instead of fixing the very serious reliability bugs, and instead of sticking in a few dozen stock photos, Paradox decided that an urgent priority was some 3D counters which they could charge extra money for. It's not good enough.
I can't recommend this game. If it was claiming to be a beer-and-crisps wargame I could overlook its historical inaccuracy. It's claiming to be a very accurate game, and it isn't. It's not accurate enough for a historical wargame, it's not fun enough for a 'quick dip in and have some fun' wargame. It's too unreliable to be any kind of game at all.