TLDRARMA (Well, Operation Flashpoint series in general) is a game I hold close to my heart because it's the best military simulator sandbox out there at the moment. You can play it seriously or just have fun with it, and that's something not a lot of people realize, they see it and go 'oh it's too simmy.' Well, strap a bunch of machineguns and artillery to a bus and play with that =p
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Aaaanyways. Should you get it if you already have ARMA 2? Depends on if you have friends who want to play ARMA 3 with you. ARMA 2 has the ACE mod which is pretty great, its main feature being the medical system but it also has things like backblast from rocket launchers and being able to interact with people, plus it has a sub-mod, ACRE, for realistic communications with radios and direct voice chat. ARMA 3 does not have this yet (And possibly might not? Some devs from ACE keep wondering why people don't think it's not coming to ARMA 3, and it's probably because they haven't made any official announcements, so who knows?)
ACE is not required, especially if you play more casually, and there's plenty of other mods and additions you can get. I remember getting a skidoo for OFP and building ramps on the Kolgujev winter map and ramping off the mountainside in slow motion just for fun. Or putting a bunch of tanks (about 100 ) a town or two away and detonating a nuke and seeing if I get hit by any of the tanks that fly out from the force of the explosion. If you have friends you can pretty much do whatever depending on waht mood you're in, I had a little group of people that play with the radio comms mod thingy but it's more casual, but you can find groups that play more seriously, or even in missions that recreate real-life scenarios.
Bohemia Interactive has a tendency to release games a year or two before they are finished, and as this game is built off of an engine from around 2001 it is still as clunky as ever, just not as noticeably so. Firing weapons on foot (which you will be doing a LOT) is more challenging and rewarding in ARMA 3 than it ever has been in the entire series, so if you like infantry based combat then ARMA 3 is a good choice. Vehicles are still janky and fairly unpolished, but they function well enough, just don't expect Steel Beasts style of simulation here. Fixed wing is kind've strange considering the size of the 'continents' but it works well enough. Rotary wing just got a DLC and update which could be worth checking out if you have the proper peripherals, but the damage model on vehicles in general is still essentially from 2001 so don't expect greatness.
Sorry, I always go on these tangents whenever ARMA is mentioned...
My opinion: wait for a sale. ARMA 3 brings some neat things to the table but ARMA 2 should keep you satisfied until the price goes down a bit. ZEUS is quite a neat thing but I believe there are similar but less polished mods available for ARMA 2 that give similar functionality.