The "three-dimensional character" formation came from playwright Lajos Egri's "three dimensions of character" model, which he discussed in his book The Art of Dramatic Writing. Egri's idea is that, in order to thoroughly conceive a character, a writer should consider three dimensions: that character's physiology, psychology, and sociology. Physiology considers how the character's body helps or hinders them in life - whether they're unusually short, tall, fat, ugly, beautiful, athletic, crippled, etc - and what adaptations and habits they develop to cope or compensate. Psychology looks basically at the character's early family life e.g. the now-familiar stereotype of the villain with the painful childhood. Sociology looks at the socioeconomic and cultural aspects of how the character developed - whether they were raised rich or poor, in the median culture or some other culture, to a prominent family or a family of peasants, etc. Taken together, these three dimensions of character should give the writer plenty of material to consider a character's background and disposition. The idea has found its way into writing character sheets. Used judiciously, it can help a writer explore a character's motives. Used indiscriminately, it can lead to a lot of unnecessary work and exhaustion.