After Tsui completed the first segment of Triangle, Lam looked at the development of it before shooting the second part, and handed the film to Johnnie To who completed the third part with its conclusion.
Each director was solely responsible for one third of the film (about 30 minute long). They did not discuss their segments with each other, and each director had a different set of writers working on each segment, the most notable being Nai-Hoi Yau, Kin-Yee Au and Tin-Shing Yip, frequent scriptwriters for Johnnie To and his Milkyway Image films. All of the segments relied on the same editor, cinematographer and music for the sake of uniformity.
Hark Tsui was the first to start production for the film, since To and Lam agreed that it was his original concept.
The first Hong Kong film made in a frame story format, Triangle had each director take charge of a film segment, bringing in their own production team and screenwriters to continue the story set in motion by the previous director.
Marks the seventh time director Johnnie To and actor Louis Koo collaborated in a director/actor relation.