The "triangle" as you term it between Tom, Daisy and Gatsby is based on the marriage between Tom and Daisy, but Daisy's evident lingering affection for Gatsby and his fervent devotion towards her. Of course, let us remember that Tom is no wronged husband in this matter. He has been adulterous throughout his marriage to Daisy, and during the time of the novel is carrying on a long term relationship with his mistress, Myrtle. The "zenith" or climax of this relationship comes in Chapter Seven, when the three and Jordan and Nick go to a hotel in the city in a terrible heat that acts as a kind of symbol of the heightened emotions. This turns into a kind of fight for Daisy's affections, and Gatsby forces her to tell Tom that she doesn't love him, but at the end of the day, Daisy is unable to leave Tom, saying:
"Oh, you want too much!" she cried to Gatsby. "I love you now--isn't that enough? I can't help what's past." She began to sob helplessly. "I did love him once--but I love you too."
It is this chapter that makes it clear that Daisy will never leave Tom, in spite of his affairs and her attraction to Gatsby. Thus this is the scene which represents the climax in the relations between the three characters.