In this paper, we first establish the expression of positive Green's function for a second-order impulsive differential equation with integral boundary conditions and a delayed argument. Furthermore, applying Legget-William's fixed point theorem and Hölder's inequality, we obtain the existence results of at least three positive solutions under three cases: p = 1, 1 < p < +∞, and p = +∞. We discuss our problem with impulsive effects and a delayed argument. In this case, our results cover second-order boundary value problems without impulsive effects and delayed arguments and are compared with some recent results. Finally, we give an example to illustrate our main results.