Optimal perturbations for the family of three-dimensional boundary layers described by the Falkner–Skan–Cooke similarity solution are obtained using a variational technique in the temporal framework. The disturbances experiencing the most growth take the form of vortices almost aligned with the external streamline at inception and evolve into streaks. In subcritical flows these can attain about twice the transient amplification observed in comparably forced two-dimensional flows. Possible connections between optimal perturbations and exponentially amplified crossflow vortices are explored.