Two-dimensional laminar flow around three side-by-side rectangular prisms placed between two parallel walls is numerically investigated at a Reynolds number Re = 100. The cross-sectional aspect ratio of each prism is b/h = 3, where b and h are the prism width and height, respectively. The gap spacing (g) between the two adjacent prisms and between the wall and the outer prism is varied from g* (= g/h) = 0.8 to 3.4. The focus is given on the influence of g* on flow structures, Strouhal number (St), and fluid forces. Three distinct flow regimes are identified, namely single vortex street (0.8 ≤ g* < 1.1), twin vortex streets (1.1 < g* < 2.1), and triple vortex streets (2.1 < g* ≤ 3.4). Lift forces on the outer prisms are attractive for single vortex street and repulsive for twin and triple vortex streets. The St being identical for the three prisms exponentially declines from 0.39 to 0.18 with g* increasing from 0.8 to 3.4. Fluctuating lift and time-mean drag also shrink with increasing g*, time-mean drag becoming higher for the middle prism than for the outer prisms. Empirical relations of St and forces are obtained as a function of g*. © 2019 Elsevier Ltd