Flanking

Optional flanking rules are in play, for player characters, as well as for enemies / allies:
"When a creature and at least one ally are within 5ft of the same enemy on opposite sides, that enemy is flanked. Each of the flanking creatures has advantage on melee attacks against it."
For this to work, you need to be precisely opposite with the enemy between you; the aim is to form a straight line through all three creatures.