Fast and accurate variance/covariance predictions are useful for analyzing the statistical characteristics of the reconstructed
images and may aid regularization parameters selection. The existing methods, the matrix-based method and its DFT approximations,
are impractical for realistic data size in X-ray CT. We have previously addressed this problem in 2D fan-beam
CT by proposing "analytical" approaches, the simplest of which requires computation equivalent to one backprojection and
some summations. This paper extends these approaches to 3D step-and-shoot "cylindrical" cone-beam CT.