Something that I've never understood. With two seats, there are three armrests. With three seats, there are four armrests (the law of fenceposts applies..).
So it appears that the arilines expect some sort of ritual combat to decide which seat will control the center armrest.
I've always felt that the armrests are meant as a barrier, to keep my neighbor OUT of my seat. You still have to deal with elbows, but that's better than having to fight for the whole seat.