Experiments are carried out to investigate on the connection between flow fluctuations and dynamics of different vorticity structures in the wake of an equilateral triangular prism placed vertically on a plane with its apex edge against the incoming flow. Fluctuations at three frequencies are found to dominate in different wake regions; the highest frequency is caused by the alternate vortex shedding from the lateral vertical edges of the prism, while the lowest frequency is connected with an oscillation of the streamwise vorticity structures detaching from the free-end. The fluctuations at the intermediate frequency are shown to originate from an oscillation of the transversal vorticity sheet bounding the recirculation region behind the body. Geometrical modifications are then introduced to successively interfere with the dynamics of the different vorticity structures and thus to identify possible interactions between the various fluctuations. Indentations along the prism vertical edges produce an increase of the mean wake width and a consequent decrease of both the vortex shedding and intermediate frequencies, whereas the low-frequency fluctuations are not affected. The latter remain unaltered even when irregularities are added to the free-end edges and the streamwise vortices are seen to roll-up with a different formation process.