Three-dimensional frontal dynamics is discussed in the context of a 3D SG model analysis with a simplified baroclinic jet as its reference state. The illustrative analysis is formulated and performed in physical coordinates instead of the geostrophic coordinates. Reasonably realistic cold front and warm fronts are shown to develop simultaneously in this model. It is found that the differential thermal advection by the geostrophic wind and ageostrophic wind components in the cold and warm frontal regions contributes to the distinct difference in the intensity of those fronts. We also present a complete analysis of the energetics of the disturbance in this SG model.