By combining the non-linear Ginzburg-Landau equations with a gauge transformation of the vector potential that accounts for the superconducting/vacuum boundary condition, the superconducting phase of a thin microtriangle under a perpendicular magnetic field is investigated. We determine the symmetry-breaking and symmetry-switching transitions that the nucleated order parameter may undergo when decreasing the temperature well below the phase boundary. It is shown that symmetry consistent vortex-antivortex patterns are stable in a broad range of temperatures and magnetic fields. The geometry of the sample also induces crossovers between vortex states unexpected for other regular polygons.