The paper presents a three-dimensional (3-D) computational study of natural convection cooling on a 3-by-3 array of discrete heat sources flush-mounted on one vertical wall of a rectangular enclosure filled with various liquids (Pr=5, 9, 25 and 130) and cooled by the opposite wall. The non-dimensional governing equations with appropriate boundary conditions are solved in the primitive variable form employing a semi-implicit finite volume formulation. The procedures are validated experimentally and numerically with published results. Computations are performed for a range of modified Rayleigh numbers from 10(4) to 10(8) while the enclosure aspect ratio varies from 1 to 20. The effects of modified Rayleigh number, enclosure aspect ratio and Prandtl number on heat transfer characteristics are investigated. The results reveal that the flow field is complex and the heat transfer from the discrete heaters is not uniform. The row-averaged Nusselt number increases with Rayleigh number in the power of 0.27. It also increases with enclosure aspect ratio in the range of 1-3 and attains the maximum when aspect ratio is greater than similar to 3. The heater surface temperature is the highest at the top-row heaters regardless of the modified Rayleigh number and enclosure aspect ratio. The effects of Prandtl number are negligible in the range from 5 to 130,