The nucleation of superconductivity in a mesoscopic equilateral triangle is studied experimentally by transport measurements and theoretically by using the linearized Ginzburg-Landau equation. The voltage and current leads, intrinsic to transport measurements. give rise to a broadening of the resistive transition R(T). However, by choosing the right resistance criterion to determine the experimental superconducting/normal phase boundary T-c(H) and by reducing the coupling between the triangle and the contacts, achieved by changing the shape of the contacts, this influence can be minimized. From the theoretical study, we found that the trigonal symmetry of the sample has a profound effect on the superconducting state in the presence of a magnetic field H leading, in particular. to the formation of antivortices in symmetry-consistent states. A good agreement between the theory and the experimental T-c(H) data is observed. (C) 2001 Elsevier Science B.V. All rights reserved.