A nearest neighbour spin pair of the quasi-two-dimensional three-state Potts model interacts with the strength J(> 0) in the xy-plane and with λJ (0 ≤ λ ≪ 1) in the z-axis. The phase transition is of second-order when λ = 0 and is of first-order when λ > 0. The dimensional crossover occurs with a change of the order of the phase transition. We study the finite-size effects of the phenomenon by using a Monte Carlo method with a multi-spin coding technique. The prediction of the finite-size scaling theory is consistent with the Monte Carlo results.