It is generally accepted that the three major dimensions of project success are time, budget, and quality. Most of the research in project planning is focused on the time-cost trade-off, and only a few papers have considered the three dimensions together. This paper describes the evolution of formulations for optimising project time-cost and quality, and continues by developing a new, non-linear optimisation formulation that better reflects the triangular trade-off structure between time,

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... , and quality. In particular, the Cobb-Douglas formula is adopted and its use is illustrated. The model is discussed, and the structure of the trade-off is analysed and illustrated.