(1) a tripartite marathon,
not to be confused with triathlon (a more or less grueling athletic competition featuring three athletic events and accompanying outfits) or, for that matter, biathlon (which combines two--often hilariously unrelated--athletic disciplines), neither of which has anything to do, at least etymologically, with a marathon (a long
distance running event or anything that takes really long and tests the limits of human endurance--and boredom), after a Greek city by that name;
(2) the viewing of a trilogy, often a movie trilogy, in one sitting (with optional intermissions and other, more informal, interruptions);
(3) the viewing of
the Lord of the Rings trilogy in one sitting.