A center pillar assembly of a 3-door vehicle may include a side outer panel, one end portion of which is formed with a quarter glass flange, an outer reinforcement member for a center pillar for reinforcing the side outer panel, in which its upper portion and its lower portion are formed in a cross section of ‘’-shape, a gusset for connecting the upper portion of the outer reinforcement member with the quarter glass flange, wherein one lateral side portion of the outer reinforcement member is connected to the gusset, and a side inner panel being arranged in an inner part of the side outer panel, one lateral side portion thereof being connected to the one lateral side portion of the outer reinforcement member and the gusset.