The goal of 3-phase method is to determine the length of an electron beam without dedicated diagnostics by varying the measurement conditions of its energy spread, through a change in the RF phase of an accelerating structure. The originality here comes from the fact that it is applied on high-brightness electron beams of few MeV generated by RF photo-injectors. It allows testing the accuracy of 3-phase method, since the length to reconstruct is known as being that of the laser pulse generating the beam. It requires establishing the longitudinal transfer matrix of a RF photo-injector, which is difficult since the electron velocity vary from 0 to relativistic during its path*. The 3-phase method in RF photo-injector has been simulated by ASTRA and PARMELA codes, validating the principle of the method. First measurement has been done on PHIL accelerator at LAL, showing a good agreement with the expected length. I will then show results obtained at PITZ with a standing wave booster and a comparison with those coming from a Cerenkov detector. Finally, measurements at higher energy performed on the SOLEIL LINAC with travelling wave accelerating structures will be exposed.