Final answer:
The third sacrifice added by the Controller is the sacrifice of truth, as depicted in Aldous Huxley's novel 'Brave New World'. This sacrifice prioritizes societal happiness and stability over individual truth and desires.
Step-by-step explanation:
The question seems to be related to the philosophical discussions often found in literature or social studies, particularly related to Aldous Huxley's novel 'Brave New World'. In the context of the novel, the Controller refers to Mustapha Mond, one of the World Controllers in Huxley's dystopian society, who often discusses the sacrifices made for the sake of the societal model portrayed in the book.
The third sacrifice that the Controller adds is a) The sacrifice of truth. The society depicted in 'Brave New World' values happiness and stability above all else, even if it means sacrificing truth and individuality. This is designed to maintain social order and prevent any disruptions that might be caused by personal desires or the quest for truth.
On a more general note, the question could also be seen as touching on Buddhist philosophy, particularly the Third Noble Truth which illustrates that suffering can be ended by letting go of cravings.