In this contribution, we use a recently developed Monte Carlo technique to investigate the impact of ferro-, antiferromagnetic interactions for different thermodynamic parameters in a three-dimensional Ising-like model. We present the role of temperature and pressure on the first-order phase-transition between a low-spin and a high-spin state and identify different response curves. The analysis of one- and two-step spin-crossover behaviour has been performed by using a self-consistent approach, because it is known that for a system with both short- and long-range interactions, the Hamiltonian does not have an exact solution. However, by constructing the distributions of the states with Monte Carlo entropic sampling and by taking into account the temperature degeneracy dependence between low-spin and high-spin states, we could solve the Hamiltonian. We provide herein a systematic comparison to our previous experimental works and as part of our continuous interest in spin crossover nanosystems, we present a size effect study. The effects of size, pressure and interactions on 3D spin crossover systems within the framework of an Ising-like model are discussed. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.