Depending on their CTFE content (from 0 to 10 mol %), poly (VDF-ter-TrFE-ter-CTFE), poly(vinylidene fluorideter-trifluoroethylene-ter-chlorotrifluoroethylene) copolymers exhibit ferroelectric (FE) or relaxor ferroelectric (RFE) properties at low temperature whereas they all present paraelectric (PE) behavior at high temperature. This thermal evolution of their electro-active properties is related to reversible crystal-crystal transitions. We studied these structural transitions for three different copolymers with various amount of CTFE (0, 4.4 and 9.7 mol %) using simultaneous SAXS-WAXS experiments along thermal cycles. We identified two types of crystalline phase at low temperature with their proper crystal-crystal transition: the first one containing all-trans conformations (orthorhombic FE phase) has a discontinuous transition towards the hexagonal PE phase, the second one which incorporates gauche disordered conformations (orthorhombic DFE (Defective Ferroelectric) or RFE) transits continuously towards the same hexagonal PE phase. For the intermediate composition (4.4 mol % CTFE), we observe the coexistence of these two simultaneous phase transitions, one discontinuous (FE to PE) and one continuous (RFE to PE), whereas only the continuous one exists in the higher CTFE composition (9.7 mol %). Relaxor ferroelectric properties are precisely observed in the temperature range of the structural RFE to PE transition [0 degrees C, 40 degrees C], highlighting the importance of this crystal-crystal transition. By coupling electric displacement - electric field (D - E) loop measurements, thermo-mechanical experiments (DSC and DMA) and dielectric spectroscopy, we propose a model to explain this RFE-PE continuous crystal-crystal transition in terpolymers.