While virtual reality and 3D interaction open new prospects for musical performance, existing immersive virtual instruments are often limited to single process instruments or musical navigation tools. We believe that immersive virtual environments may be used to design expressive and efficient multi-process instruments. In this paper we present the 3D reactive widgets. These graphical elements enable efficient and simultaneous control and visualization of musical processes. Then we describe Piivert, a novel input device that we have developed to manipulate these widgets, and several techniques for 3D musical interaction.