The two-dimensional (2-D) direction-of-arrival (DOA) estimation for incoherently distributed (ID) sources always has a high computational complexity because it depends on multidimensional search. To reduce the complexity, we propose a new method based on three closely parallel unity linear arrays (ULAs). This method derives the approximation of three received ULA vectors under small angular extension from first-order Taylor approximation. The derived vector approximations are rotationally invariant, which enables one to estimate 2-D DOAs of ID sources via propagator. This procedure is more efficient because no multidimensional search or eigen-decomposition is needed. Simulation results testify to the performance of the proposed method.