Especially in recent times, modern societies are more and more longing for substitution of pollutive technology (coal-fired power plants, gasoline driven automotives,...) by eco-sensitive solutions. This balancing act, between fulfilling the need of current energy expenditure and expected increasing power demand should be satisfied and optimized via solutions as e.g. renewable energy, the smart grid vision, e-mobility etc. Therefore, a grid harmonic free three-phase AC-to- DC conversion is highly requested as a lot of today-s technologies are assuming a DC power supply and/or at least a common DC-link (battery systems, motor applications, miscellaneous DC power supplies,...). Three-phase rectifier applications also play an important role for e.g. e-mobility fast charging stations. Such stations often use passive three-phase rectifiers with adjacent passive filter (low-cost, simple, robust). Due to a high reactive power demand and higher order harmonics (which mainly result in a low-cost and volume optimized design of the passive topology- THDi typ. 48%) such systems tend to impair the stability and highly pollute the grid. These mentioned drawbacks can be overcome or at least reduced if active rectifier solutions are implemented. Commonly known active rectifiers, however, do not allow the extension of such a passive system, and thus in general assume the substitution of the total AC- to DC-conversion path. The already manufactured passive system is hence not reusable. An optional upgrade which allows both, either a high efficiency or unity power factor and low harmonic input currents (on demand) therefore might appear as attractive solution for such already existing passive systems. From this reason, this PhD-thesis is engaged in such an enhancement for passive three-phase rectifiers with DC-side smoothing inductor by employing the third harmonic injection concept. Several promising solutions are discussed and analytically described. The most promising solutions are selected for laboratory prototype verification. The implemented circuits should confirm the proposed advantages of such a hybrid system (improved efficiency, low harmonic input currents THDi<5%, unity power factor, no high frequency common-mode voltage with switching frequency at the output of the total system). Within the scope of this PhD-thesis, not only novel hybrid power electronics concepts have been proposed, but also dedicated control concepts have been analyzed and verified. Furthermore, the laboratory prototypes also considered different important practical aspects as e.g. optimized input filter design due to exploitation of mains impedance, design of a highly dynamic control (voltage/current), active voltage balancing, comparison of designed and implemented components via thermal measurements, optimized injection inductors, advanced cooling concepts (optimized air cooled) and unbalanced mains input voltages.