Recently high performance electric and optoelectronic devices based on organic semiconductors have been demonstrated, such as organic light emitting diodes (OLED) and solar cells. These devices show promise for low-cost, large-area and flexible devices. On the other hand, rapid progress of organic light-emitting transistors (OLET) has been made in recent years. OLET is a new light-emission concept, providing planar light sources that can be easily integrated in substrates like silicon, glass, paper using standard microelectronic techniques. We present the numerical simulator that we have developed to solve charge transport equations in OLETs. In the three-dimensional model, the Drift Diffusion system is extended to include the effects of external magnetic fields and of anisotropic mobilities, the presence of trap-assisted phenomena and the generation of singlet and triplet excitons. Non-standard discretization methods have been devised to deal with these non-classical features.