Arma 3 is an all around game, which allow you to discover a lot of opportunities to play this game.

Singleplayer campaign:
When you have installed your game, I will suggest you start your journey with the Arma 3 Campaign, it allows you to learn the basic in Arma, such as controls and how the environment pushes your boundaries of your character ( Stamina ) and will make sure you down hit the target before you catch your breath.
The content and story of the campaign is great and you'll have to work your way in to find the connecting thread of it all.

The DLC's:
The DLC's have a certain amount of content such as:
▶Maps
▶Gear
▶Vehicles
▶Campaigns

Online:
Online game servers is wide and are ready to be played on, the playerbase is high and the people inside are ready to help you, if you need it.
There are certain amount of gamemodes for you to play.
If you like PVP, then you should go ahead and play on King of the Hill servers, where you and your team must capture a zone and protect it against your enemies.
If you seek teamwork or a common cause, the you should head to Invade'n'Annex servers, where you'll have to work together with your teammates and capture a zone infested with enemies and complete side missions while during the main task.

Workshop:

The workshop is an endless library for you to customize your game with varies gear, vehicles and engines and suit your game as you want it to be.


I personally give this game 5/5 ★★★★★ for the mass playerbase and opportunities for how to play the game.

I hope this has cleared out your reason to play this game.