Falling Man, by Don DeLillo, is a story which takes place immediately following the tragedy of September 11 in New York. The novel follows several characters, including one man, Keith, who walks away from the tragedy covered in someone's blood and suffers the effects of living through this tragic event. He and his wife, Lianne, never recover from the trauma.
The novel is divided into three parts, as noted in the question; each division carries a man's name as its title. Each of these names is a representation of something else associated with the act of terrorism perpetrated on America on that fateful day.
"Bill Lawton" is not a real person; instead this is name the children hear and repeat (instead of "Bin Laden") as they listen to the news after the event. Their fear is real but misplaced, and the implication DeLillo makes here is that terrorism is not just foreign or unfamiliar. By "Americanizing" the name, DeLillo implies that terrorists can come from within as well as from without.
"Ernst Hechinger" is the real name of the man Lianne's mother is living with. He is known now as Martin Ridnour, but it is clear that he was once a radical activist somewhere in Europe. Despite her fears about terrorists, Lianne's mother seems perfectly content to have one for a lover. This irrational acceptance of Ridnour in the face of her fears highlights the irrationality involved with terrorism and terrorists.
"David Janiak" is the real name of a performance artist commonly referred to as "Falling Man," though he never actually falls because he is harnessed to the structures from which he drops. Falling Man wants to represent those who fell to their deaths in the tragedy; however, because he never actually falls, he fails to truly recreate the events accurately. His life is spent not falling and reminding people of the difference between that and the real falling which was done on September 11.
All three of these title characters are somehow false and misleading, perhaps representing the inability of the human mind to fully comprehend and rationalize what it experienced on that tragic day.