While DnD 5e no longer includes flanking as a standard rule, they have an optional rule that covers it. I've not played earlier versions, but I imagine it's very similar (if not identical) to the standard rule that existed in earlier versions. Here is the optional flanking rule from DMG p. 251:
When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy's space, they flank that enemy, and each of them has advantage on melee attack rolls against that enemy. – (DMG p. 251)
So if a medium creature (taking up just one square) has three adjacent enemies, two of whom are on opposite sides or corners, do all three have advantage, or just the two who are flanking?
A B A
B T OR A T A OR B T
A A etc.
Where "A" and "B" are allies and "T" is the target creature.
In each of these examples, "B" is not causing the target creature to be flanked, as there isn't another creature on the opposite side to "T" from it. Does B still get advantage?
Put another way, could we say that if a creature is flanked (see above) then all melee attacks against them are made with advantage? Or would it be better to say if "they flank that enemy, […] both of them [have] advantage on melee attack rolls?"
Best Answer
When dealing with Medium creatures, yes, it might read more easily to say "both". The "each of them" becomes important when dealing with Large or bigger creatures, which take up so much space that multiple creatures can fit side by side on one end.
The rule, from the viewpoint of the attacker, is basically:
If yes, you have flanking.
For an example of when the "each" comes into play, see below diagram of three Allies flanking a Giant. In this situation, all three Allies attack with advantage due to flanking, because "Am I adjacent to an enemy and is there an ally adjacent on the opposite of the creature" is true for all three.
On the other hand, in this situation, that doesn't work.
While the top and bottom attacker have advantage due to flanking, the one on the right does not.