There aren’t any written rules about who is entitled to which armrest on an economy flight, but there are some unwritten ones.
If you’re in the aisle seat, you get the armrest closest to the aisle.
If you’re in the middle seat, you get the two middle armrests.
If you’re in the window seat, you get the armrest closest to the window.
Yes, that means that the person in the middle gets twice as many armrests as the person on either side, but that person is stuck in the middle seat. They don’t get any of the perks that come with the window or the aisle seats, such as “the ability to get up and use the lavatory without having to ask someone else to move” or “the ability to control the windowshade.”
(Not that you aisle seat people would refuse to get up when asked, or you window seat people would refuse to lower the shade if the sun was in someone else’s eyes. Right?)
If the person in the middle is not using their armrests, the people seated on either side have the option of claiming those armrests for themselves. But give the person in the middle a chance to take advantage of those few extra inches of armrest space first, ok? Wait until you hit cruising altitude, or something.
And middle seat people: if those armrests need to go up for the comfort of the passengers on either side of you, leave ‘em up and don’t fuss about it. Economy class is miserable for everyone, but at least most airlines offer a whole pile of TV shows and movies for you to watch while you try not to think about how cramped you are.
Rest your arms wherever they can find a space, whether that’s on your lap or on the tray table in front of you, and sit tight until you reach your destination.