Three-dimensional spatially growing perturbations in a two-dimensional compressible boundary layer are considered within the scope of linearized Navier-Stokes equations. The Cauchy problem is solved under the assumption of a finite growth rate of the disturbances. It is shown that the solution can be presented as an expansion into a biorthogonal eigenfunction system. The result can be utilized for decomposition of flow fields derived from computational studies when pressure, temperature, and all the velocity components, together with some of their derivatives, are available. The method can be used also if partial data are available when a priori information may be utilized in the decomposition alogorithm. Properties of the discrete spectrum for a boundary layer over a cone with an adiabatic wall at the edge Mach number 5.6 is explored. It is shown that the synchronism of the slow discrete mode with acoustic waves at a low frequency or a low Reynolds number is primarily two-dimensional. At high angles of disturbance propagation, the fast discrete mode is no longer synchronized with entropy and vorticity modes.