We study the stability of laminar wakes past three-dimensional rectangular prisms. The width-to-height ratio is set to W/H = 1.2, while the length-to-height ratio 1/6 < L/H < 3 covers a wide range of geometries from thin plates to elongated Ahmed bodies. First, global linear stability analysis yields a series of pitchfork and Hopf bifurcations: (i) at lower Reynolds numbers Re, two stationary modes, A and B, become unstable, breaking the top/bottom and left/right planar symmetries, respectively; (ii) at larger Re, two oscillatory modes become unstable and, again, each mode breaks one of the two symmetries. The critical Re values of these four modes increase with L/H, reproducing qualitatively the trend of stationary and oscillatory bifurcations in axisymmetric wakes (e.g. thin disk, sphere and bullet-shaped bodies). Next, a weakly nonlinear analysis based on the two stationary modes A and B yields coupled amplitude equations. For Ahmed bodies, as Re increases, state (A, 0) appears first, followed by state (0, B). While there is a range of bistability of those two states, only (0, B) remains stable at larger Re, similar to the static wake deflection (across the larger base dimension) observed in the turbulent regime. The bifurcation sequence, including bistability and hysteresis, is validated with fully nonlinear direct numerical simulations, and is shown to be robust to variations in W and L in the range of common Ahmed bodies.