  We study three antiferromagnetic formulations of the O(3) spin model in three
dimensions by means of Monte Carlo simulations: 1. a two parameter $\sigma$
model with nearest and next to nearest neighbors couplings in a cubic lattice;
2. a face centered cubic lattice with nearest neighbors interaction; 3. a cubic
lattice with a set of fully frustrating couplings. We discuss in all cases the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first order.
