If three guys are attacking one guy, they should be able to shift a little so that when its your turn to attack you can either just attack with flanking and then shift, or shift to be able to attack with flanking.
In other words, ABC are attacking M.
A and B are flanking (A attacks)
B and C are flanking (B attacks, and shifts)
C and B are flanking (C attacks and shifts)
etc...
In other words, ABC are attacking M.
A and B are flanking (A attacks)
B and C are flanking (B attacks, and shifts)
C and B are flanking (C attacks and shifts)
etc...