We develop a three-dimensional theory of coherent synchrotron radiation in steady-state. Starting from the Hamiltonian in the Courant-Synder theory in a curvilinear coordinate system, we find that the transverse force is essentially the Lorentz force with an additional term: $-e\phi/\rho$ in the bending plane, where $\phi$ is the potential and $\rho$ the bending radius. The potential term partially cancels the centrifugal force, resulting in a residual wakefield: $-\Lambda\delta(z)/\rho$, where, $2 \leq\Lambda\leq 4$ and $z$ is the longitudinal coordinate. The measurement in the bunch compressor supports the sign predicted by the theory but with a smaller value. Show Partial Abstract