In this study fluidic oscillators are examined for separation control purposes over base flaps attached to a three-dimensional bluff body which resembles the rectangular shape of a tractor-trailer model. Fluidic oscillators continuously emit a high velocity jet which is spatially oscillating along the flaps’ span at high frequency. These flow control actuators successfully prevent flow separation over the base flaps, thereby significantly reducing the model’s drag. Even when accounting for an overestimated momentum input, the total drag is reduced by 16% without any system or setup optimization. It was found that the drag reduction improves when increasing the flaps’ length at a constant actuation level. Furthermore, a larger spacing between adjacent actuators reduces the minimal momentum input required to attach the flow. Instead of the momentum coefficient, the velocity ratio governs the actuation intensity for changing actuator spacing. A velocity ratio of five yields the most efficient results. Some qualitative flow field observations demonstrate the actuators’ effectiveness in preventing flow separation over the entire flap. Additionally, vortical structures are observed in the vicinity of the jets’ exit, which are suggested to be the reason for the oscillators’ superior performance.