This article addresses the problem of damage localization in an isotropic plate using guided waves. This localization is based on Time of Flight measurements thanks to an array of three sensors arbitrarily distributed on the structure. First, an exact solution of the Time of Flight problem is proposed. Then, the exact analytical Cramer-Rao Bounds, expressed as a function of the actuator and the sensors locations and the Signal to Noise Ratio of the measured signals, are presented. Statistical performance of the method is illustrated by means of Monte-Carlo simulations and compared to the Cramer-Rao Bounds. Time delays are estimated both with an Hilbert Transform method and a Maximum Likelihood Estimator. Results show that the performance of the algorithms is in agreement with the theoretical variance if Signal to Noise Ratio is low enough. These Cramer-Rao Bounds provide a way to determine the optimal position of a sensor array for a given a-priori accuracy.