When doing three charged attacks in quick succession, the third charged attack charges briefly then the charging stops and the character performs a weird animation change before swinging the sword. I believe it does the normal attack damage and the stamina consumption matches that of a normal attack.
Video of it (+ player logs): Unique Download Link | WeTransfer
This is a 100% replication rate and so far does not work with any other weapon I own (other greatswords, spears, one-handed swords…).
It is also possible to obtain the same result but without the first two attacks being charged attacks, however I found it to be less consistent.