On HoI3 AI
Since I mostly play a modded game (e.g. Downfall), or played MP games, I can't really remind myself of the vanilla AI. But I can say that AI military behavior in HoI3 is generally better than in HoI4 (hence why I ditch HoI4 for the time being).
Main HoI3 strengths
- AI has a very good air units handling
- AI does quite less units shuffling but still some
- General military behavior seems fine
Main HoI3 flaws (like posted before by many) :
- AI has a real hard time protecting itself against amphibious invasions behind its lines (as it will overlook them or react too abruptly to them by sending troops backward
- AI relies a bit too much on either small fleets, or very large one... (so in one case, you kill them quickly, in the 2nd...well, they suffer a huge penalty)
- AI tends to sometimes stack too many units in some provinces (hence getting a penalty for over stacking)
- AI detects close flanking attempts but not grand flanking attempts (those done like 5 provinces away)
- AI manages partisans poorly
- Basic scripts are not optimised and this induce a lot of issues (bad division composition, bad production stances in some circumstances, bad trades and trade lines management, bad diplomatic stances, etc.)
To overcome these mains flaws, you need some modding (for AI optimisation) and to play at higher level than NORMAL (above all to overcome AI auto-inflicted stacking penalty and make the game harder to a Human... btw you can mod those penalties to make it more challenging instead of only targeting IC).
What mods can do :
Depending on the work done, good mods can change general AI behavior and induce changes to AI military behavior (even though you can't change the behavior itself in the latter case).
General behavior: you can mod the production behavior, including production and technology research optimisation (adapting production to specific situations, divisions’ composition, recruiting laws, techs priorities or sequence, to optimise the use of leaderships points, etc.). All in all, this can make a big difference on the field as AI will put better units on the map, at the proper time (but it can still use them cluelessly, like Kovax and others pointed out...). You can also protect AI against itself by optimising its diplomatic stances (with whom to trade and not trade, to remove trade lines in time to save some transports, to DoW whom when or when to selfprepare against Human DoW, and so on). A lot can be done
Military behavior: AI reacts to VPs. Low level (1-3) induces swift reaction from him, while higher value (4+) seems to trigger a complex response
For instance, I'm currently testing that in my mod and I have great expectations. Still waiting to see the results. In my current game tough (testing 1.37), the changes I made to VP did induce some changes in the way AI reacted to my attacks. So far, so good. But only a complete game will tell me if I did something meaningful.