Hmmm, I suppose that's a bit of a tough question.

I'd say it rates as semi-difficult, mainly due to the realism and expanded coverage you get. It is inherently more "sim" than a normal Xbox or Playstation game (read: Call of Duty, Battlefield etc.). ARMA 3 is as close to a modern combat "simulation" as you're likely to get. While the game has a campaign, the franchise has always been one thing: a gigantic sandbox. By gigantic, we mean...gigantic. There are two maps provided, the larger of which features over 200 square kilometers of actual terrain. It's a fake gigantic post-colonial island in the Mediterranean (think Corsica/Malta?).
Here is a picture to demonstrate the scope (compared to other games shown below)

Due to the size of the map and the simulation nature, engagements tend to happen at more realistic differences (infantry engagements at 200-400 yards, tanks can engage each other at over 2800 yards etc.). The game includes aviation units (helicopters and ground attack aircraft), boats (attack boats, patrol boats, combat divers, submarines etc.), infantry, civilian vehicles (cars, trucks, etc.), light armored vehicles (MRAPs, humvee-style vehicles etc.), artillery (self propelled guns, MLRS systems), armor (MBT, armored anti-aircraft tanks, BMP-style vehicles etc.). Weapons and equipment are 100% adjustable, with packs/belts/backs holding X amount of gear.
Weapons can have suppressors, PEQ (lasers), magazines, optics etc. installed on them. Weapons have accurate select-fire controls. Most combat optics feature magnified optics and single-power optics which you can switch between on the fly. You can adjust the zero in 100 yard increments as you wish etc. Smoke grenades, rifle grenades, flares, etc etc etc. The list of equipment, options and material is pretty mind-boggling.
Putting all of this material into the hands of the gamers is pretty intimidating...but with the advent of Steam, you can download pre-made missions from other players. You simply click one button and it downloads the mission to your computer.
The game is definitely far more difficult than Battlefield of Call of Duty because of the increased realism. Carrying too much kit? You'll get tired quick - try aiming your rifle when your heart is pounding and you can't catch your breath. Rifles have real genuine recoil (ie. shooting while standing or kneeling is tough beyond 50 yards). Distance becomes important too. Unlike the common FPS, you can and will be shot at from 300+ yards away. The high-power sniper rifles can hit out to 2000+ yards just like in real life.
Now a disclaimer: I don't play the game multiplayer/adversarial. I play co-op missions with buddies of mine against the AI which I've scripted into missions. It provides a lot of pants-pissing moments (a tank firing near you will damn near blow out your ear-drums). It's not uncommon for a single mission of ours to run for 2 hours or so - only engaging a couple of dozen enemies.
Add on top of this a fully realistic weather/time system and you can see you'll get immersed in it pretty quick (fog, lightning, rain, wind, waves, etc.). It's a phenomenal game if you can sort out missions. It's just not for the feint of heart (or the person who wants to pick up a controller, kill a dozen people in 5 minutes and then go on about his business).
