We present GENESIS-RT, a Virtual Reality platform conceived for musical creation. It allows the user to (1) interactively create physically-based musical instruments and sounding objects, and (2) play them in real time in a multisensory fashion, by ways of haptics, 3D visualisation during playing, and sound. The design of this platform aims for full physical coupling, or instrumental interaction, between the musician and the simulated instrument. So doing, it differs from both traditional Digital Musical Instrument architectures and Virtual Reality system architectures. By presenting our environment, we discuss several scientific underlying questions: (1) possible ways to manage simultaneous audio-haptic-visual cooperation during real time multisensory simulations; (2) the Computer Aided Design functionalities for the creation of new physically-based musical instruments and sounding objects, and (3) the synchronous real time features, in terms of software and hardware architecture. Finally the article reviews a series of exemplary models and instrumental situations using the proposed platform.