Let's say there are 3 in the party attacking a single enemy. Two of the party are flanking the enemy, the third is right next to one of the other allies (Enemy E, players with number, blank space X)
XXX
1E2
3XX
Does player 3 get the advantage of flanking as the enemy is flanked, or will that only apply to players 1 and 2?
The flanking rules say:
To flank a foe, you and your ally must be on opposite sides of the creature. A line drawn between the center of your space and the center of your ally's space must pass through opposite sides or opposite corners of the foe's space
That is, for you, the attacker under consideration, it matters if you have an ally on the other side. The creature in the middle does not gain some kind of "flanked" status flag if it is flanked by some other creatures. You check for each creature around it if the flanking conditions are fulfilled.
In your example, you can't draw a line from the center of player 3 and cross both opposite sides of the enemy square with any other ally. Thus player 3 does not get the advantage of flanking.
There is even an image in the rules that explains this situation:
Here Merisel is your player 3, and Valeros and Kyra are your players 1 and 2. They flank the creature, Merisel does not.
