The electron transfer (ET) process has been studied in a three sites system: the donor and the acceptor of electrons separated by a bridge. We have considered an isolated system in order to understand the characteristics of the process itself without introducing a bath. The ET process has been studied considering both the electronic and the vibrational part. This complete vibronic analysis has been done in a three electronic-n vibrational model. Two questions are put into evidence. First of all we have analyzed the relevance of the vibrational part in modifying a pure electronic description both in the electron transfer time and in the site populations. A second aspect underlined is the difference between a tight-binding system (donor and acceptor without a direct coupling) and a full coupling one. The drastic difference between these two cases has been rationalized.