Who is the serie killer that they usually mention. The name has three lettars. Like AFK or JFK.

The Btk killer. There is a movie out on dvd about him.

I believe you are referring to Dennis Rader, a.k.a. the BTK killer. His modus operandi was to Bind, Torture, and Kill ( BTK ) He sent boastful letters to police and news outlets during the period of time in which the killings took place. He took a long hiatus in killing in the 1990's and began sending letters again in 2004, leading to his arrest in 2005.
I hope that helped ???

BTK, maybe?