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This paper introduces a formulation for design of Fast Output Sampling (FOS) controllers for three-time-scale systems. It is shown that the FOS control gain for a three-time-scale system can be obtained by combining the solutions of the three subsystem problems, obtained separately. Since three smaller order subsystem problems are to be solved in lieu of one high order problem, numerical ill-conditioning is completely avoided. Techniques for block-diagonalization and composite control of a three-time-scale system are also discussed. The proposed method is applied to the problem of spatial control of advanced heavy water reactor (AHWR). The model of AHWR is decomposed into three subsystems respectively named “slow,” “fast 1,” and “fast 2,” and separate subsystem control problems are cast from which an FOS controller for the original system is derived. Efficacy of the controller thus obtained is demonstrated through dynamic simulations. The controller thus designed employs only the output information to achieve arbitrary pole placement.