We present a self-consistent method for the evaluation of the electronic current flowing through a multiterminal molecular wire. The method is based on Büttiker-Landauer theory, which relates the current to one-electron scattering probabilities. The scattering problem is solved using a tight-binding form for Schrödinger’s equation that incorporates a self-consistent evaluation of the electrostatic potential in the region of the molecular wire. We apply the method to a three-terminal molecular wire connected to metallic leads. The molecular wire is a π-conjugated carbon chain with thiol end groups, self-assembled on the cleaved edge of a multilayer of alternating thin metal and insulating films. The ends of the chain bond to two outer metal layers that act as source and drain, and the chain bridges a third (inner) metal layer that acts as a gate. We show that transistor action should occur in this device if on the surface of the metal gate there are adsorbed atoms that acquire charge as the gate voltage is increased, thereby enhancing the interaction between the gate and molecule and creating a strong potential barrier that hinders electron flow along the molecular wire. We find that electronic solitons play an important role in the response of this system to applied voltages.