This paper presents the numerical investigation of the effect of thermal buoyancy in flow past three circular cylinders in staggered configuration in a vertical duct. The heat transfer and flow characteristics are studied by varying the Richardson number Ri, cylinder spacing S, and the Reynolds number Re. The study reveals that for a certain Richardson number (Ri = 0.4) vortex shedding from the rear cylinders is suppressed by the accelerating flow caused by aiding buoyancy and modified flow patterns around the cylinders. The critical values of Ri were found to vary for different spacings indicating the influence of the channel walls on stabilizing the flow. The streamlines, vortex structures, and isotherm patterns for different S (0.75, 1, 1.25), Ri (–4 to +4), and Re (100, 180) are presented and discussed. Study also reveals that the Nusselt number Nu and the drag coefficient CD are strong functions of S and Re. At Re = 100 and S = 1, with a decreasing strength of opposing buoyancy, vortex shedding behind the side cylinders starts getting suppressed and CD reduces from a value of 8 to 3.2 for Ri changing from –4 to 4. The coefficient CD reduces with decreasing strength in opposing buoyancy (CD = 4.2 at Ri = –4) and increases with aiding buoyancy (CD = 4.8 at Ri = 4) due to the change in the recirculation zone behind the middle cylinder. The surface-averaged Nu increases monotonically from 4.9 to 7.45 with Ri for the middle cylinder. There is a monotonic increase in Nu with aiding buoyancy but only for the side cylinders heat transfer is reduced with decrease in the opposing buoyancy. The coefficient CD is a weak function of S. A change in CD with a change in Re is significant in aiding buoyancy compared to the opposing buoyancy. © 2020 by Begell House, Inc. www.begellhouse.com