Ok, let's begin with the modest task of describing "What the fuck is this game anyway?"
Arma III is the third sequel to Operation Flashpoint, I know what some confused people are about to say who don't the backstory to that "But, Operation Flashpoint had a Sequel and a standalone expansion!"
This was because Bohemia interactive actually made the first Operation Flashpoint, however, when they split from their publisher, Codemasters (who later made OFP 2 and Red River), they got to keep the rights to the name of the series. Bohemia, not willing to get fucked out of their legacy so easily, Renamed Operation Flashpoint: Cold War Crisis to Arma: Cold War Crisis as they wanted to continue on the same style of game, though this name change wasn't done until fairly recently to fully disown itself from Codemasters vision of how to continue the franchise. (I guess Arma: Intellectual Property Crisis would've been a better name in hindshight.)
Now then, as for the Arma franchise itself, let's describe what it is.
Arma is a tactical shooter/milsim series that goes for an authentic/semi realistic approach to the gameplay (well, at least up until 3, more on that later), and focuses heavily on strategy and tactics as a team more then many shooters out there to accomplish tasks set in the game, some of the features that are seen throughout the series are things such as.
-Massive maps with detailed terrain and biomes. with lots of matching themed towns and cities to play on.
-Destructible buildings and object penetration and I don't mean the watered down destructible or penetratable that the Battlefield and CoD series and other series advertise where you can probably survive the Tsar Bomba hiding inside an old adobe shack or behind a cast iron fence, I mean "even in a large building, if something like a 155mm arty shell barrage lands on you, you and anyone inside is likely getting pink misted while a significant portion, if not all of it collapses" destructible/penetrateable. you can even kill crew members of vehicles, from not armored whatsoever to even the toughest vehicles without having to destroy them with this system. (example, taking out the gunner and driver of a tank with a single well placed 120mm Sabot and repairing it to use against them is quite satisfying.)
-I guess to go further on the above, Vehicle physics are really good, you can easily be killed driving like an idiot and damage suspensions and wheels if you're not careful, but the way vehicles handle both on and offroad feels really satisfying, as I mentioned, if your vehicle is damaged, it will depend on what it's damaged by and where you're actually hit to determine it as the game does feature realistic parts of the vehicle that can be damaged separately, you can usually set an AFV ablaze with a well placed shot to the side or rear of it's engine/transmission deck, or detonate it's ammo rack if you know where to aim, on the other hand, firing and only hitting things like tracks will just do damage to that, armor on all sides is realistically done, as is bulletproof/aluminum based glass on aircraft glass and certain military vehicles (there's civilian/insurgent ones for reference on that) which can take many types of small arms fire. lastly, opening/closing vehicle doors on things like transport helos actually does matter aswell as turning in/out to see as a driver, unlike other games where you get some mystical bullet shield while doing so.
-Ballistics are somewhat good in the sense that you do have to adjust for hills, gravity, and wind, and rangefinding/zeroing in your sights is important in mid-long range engagements, there's also the fact that, much like in reality, gunfire tends to do that whole "kill you dead" thing very fast, body armor will generally only take 1, maybe 2 shots for you if you're lucky depending on hit location, sometimes none at all on the upper torso. Headshots are usually instantly fatal unless its at long range or a pistol and you have a heavy helmet on that manages to take it (if it hits your exposed face it won't do shit.) damaged limbs will impair you, untreated hits to the arm you manage to survive will affect things like accuracy, hits to the legs will slow you, or even force you to prone if untreated, your screen will also flash with blood marks and pain signals making it difficult to see, this isn't a game where you can just get hit 4 times and act like nothing happens until your health bar magically hits zero. recoil is pretty accurately modeled, aswell as the bipod feature.
-Aside from kinetic weapons, explosive based weapons have a very large kill/injury radius like in reality in this game and can temporarily deafen you if you're too close but you're below a hill or something to shield you from the blast/fragmentation, being outside the kill radius of a grenade but still within range to be injured is going to make you a sad panda, as it's likely to shower you all over with shrapnel and leave you incapacitated regardless. (medics are a godsend in this game.) I suppose lastly, gunshots have a cool little detail where the sound of them from a distance depends on where you are, and any terrain or other barriers between you, which is nice.
-logistics are important in this game, you have a realistic loadout on both infantry and vehicles (in both ammo and fuel) for the most part, don't outrun your supply lines.
-The weight of your kit directly affects how far you can sprint and how fast you regain stamina, and having low stamina also affects how easily you can steady your weapon, so there's actually a reason to load lighter if you can.
-The Zeus system they added is lots of fun, and allows a game master to keep a mission going on the fly.
Now then, there are some downsides as I've pointed out in the past, many of these started to appear in Arma III when they went from slight past-present day settings, I'll get those out of the way now. (warning, autism and nitpicking below)
Arma III, unlike the previous 3 games, is set in 2035, this was apparently to freshen things up, but it gave Bohemia a sort of hand wave ability to do alot of dumb shit, let me list the ones I find particularly annoying.
-I guess since they ran out of OPFOR ideas, the primary antagonist is some super strong Iran in this game who recently invaded Israel and is trying to get a foothold on the fictional Mediterranean island nation Altis (where the game takes place, alongside the smaller secondary island Strattis), that sort of bothers me because there's no way Iran could be as strong as they're portrayed in this game in a mere 20 years unless like, Russia and China dumped a goddamn massive amount of funding and equipment into them for some reason.
-The equipment they chose is very odd, many of the AFVs for BLUFOR (NATO) is literally Israeli equipment, aside from my slight bias towards Israeli gear (not because of the nation itself, but just because know nothings fanboy the shit out of it like they do with German gear), this makes no goddamn sense as Israel has never been a NATO member. this is including, but not limited to, the Merkava IV, the Namer, the Negev NG7, (this is OPFOR, likely caputred from Iran's Israel invasion) the Mini spike launcher, the TAR-21, and some other shit I'm too lazy to complain about.
While I said the ballistics are somewhat good, "somewhat" would be the key word, as the main service rifle for NATO are based on some godawful abomination child of the Magpul Masada and the Robinson XCR chambered in 6.5x39mm Grendel, with cased and caseless versions. (they don't call it the Grendel though, likely for copyright reasons.) because I guess Bohemia bought into reformist talk of gerpersherrs being totally cool and awesome, the Iranian Khaybar is also strangely chambered for this. The primary sidearms are the Walther P99 and MP-443 Grach chambered in 9x21mm, now, I know there actually is a 9x21mm round chambered for many 9mm Luger pistols intended for some countries where military cartridges are forbidden for civilians, but this makes no sense for a military gun. There's also a 9x21mm Russian round that I know have been used in a couple PDWs, but why would NATO be using this? also, for real life calibers that are just sort of "there", they're often underpowered and in as "old" insurgent weapons, which is a shame.
I could go on more but meh.
What's my overall thoughts on Arma 3? Well, I don't like the change in settings or adding in alot of hypothetical or outright implausible future items, but the graphics, sounds and physics are really good, mainly compared to the older games, the squads system is really nice, it still focuses on tactical and strategic, highly team focused combat over an extremely large scale, the damage modeling and "what can actually damage what?" question is answered for the most part properly, Overall, it's far from perfect, but it's come a long way and eliminated alot of the bugs that used to plague it, and mostly, it's very moddable, and a modded Arma 3 is night and day compared to unmoddded Arma 3, I'll post a list of some of the best after this.