The third film is literally titled Sethurama Iyer CBI (2004). However, many fans use the same keyword to refer to the first film (Oru CBI Diary Kurippu). If you want the origin story, start with the 1988 movie.
Look for releases labeled "Human Translated" or "DVD-Rip subs." sethurama iyer cbi with english subtitles full
"I am CBI Officer Sethurama Iyer. And I know you did it." The third film is literally titled Sethurama Iyer CBI (2004)