Would I recommend ArmA 3? Well that depends.

First things first the game is a "milsim" a military simulation so it is to be expected that if you've played shooters such as CoD or battlefield before and like the fast pacing you most likely wont enjoy ArmA. Though on the other hand if you are looking for something more realistic with huge maps and combined arms combat and actual military strategy ArmA has you covered.

Let's get the singleplayer out of the way. The singleplayer in ArmA 3 is ok, it will show you the basics and some ♥♥♥♥♥ basic "strategic" know how. However the base game campaign storyline is mediocre. it's not bad it's just not that good either, but that isn't really the point of ArmA.

Then we get to the multiplayer. The multiplayer is really the games bread and butter. It has basically everything there are less realistic game modes such as King of the hill and such, then there are more realistic ones like invade and annex. However if you want a true Milsim feel you should to join one of the milsim communities and join their OPs. Though the main thing about ArmA 3 MP is that depending on what you want play with the same sort of people. Want a true fullblown milsim join their group, wont be as easy but a LOT of fun once you get there. Want milsim content but at a less realistic level just find a group that does such OPs. If you want to fight as a resistance movement play Antistasi. There really isn't an OP type that isn't routinely ran by some group you just need to find said group.

Another thing that is keeping ArmA alive after all these years is modding. No milsim community plays the base game. People push for more realism and scenario types so there are mods for that. The base game ArmA 3 is set in 2035 want an OP with modern day gear like M4s, AKs, T-72s and M1s try the RHS or CUP mods, want to go roll over the eastern front in Tigers, T-34s, or just as a normal rifleman try the Iron front mods, or perhaps vietnam, there is the unsung war mod for that. Even Sci-fi mods Excist such as Warhammer 40k, Halo, Star Wars and so forth.

Also the most important form of mods in my opinion are the "Realism enhancing" ones like ACE or TFAR. ACE adds an enormous amount of thing to list a few an improved medical system with different medicine, bandage types and IV, it also completely reworks ballisticks to the level one would expect from say Sniper Elite with not just distance affecting bullets but also wind, temperature and air humidity. TFAR adds a new radio system that works through TeamSpeaks 3D audio. It makes basically all combat situations feel far more alive when its distance based how well you can hear others, or you have to communicate through radios.

Then the DLC. The DLC in ArmA 3 isn't required. Personally I haven't used most of the DLC assets even though I own them. I mostly utilize mod assets due to the zeus scenarios we run with our friend group. Also the way bohemia deals with DLC vehicles is also great. You can enter DLC vehicles as a passenger no problem so even if DLC vehicles are utilized it won't limit you getting a ride in them.

TLDR: If you want a realistic milsim shooter and can put the effort of becoming a part of a milsim group and utilize mods you can have endless amounts of fun, however if you just want a shooter you can just open the server browser and join a game I would instead recommend squad.