The 3D Energetic Electron Spectrometer (3DEES) is conceived as a compact and modular science-class spectrometer optimized for measurements of angle-resolved high-energy electron fluxes in the heart of the radiation belts. The covered electron energy range extends from about 0.14–10 MeV, with an adjustable number of measurement channels between 16 and 32 (set to 24 bins). It also allows for the quantification of proton fluxes in the energy range 2.5–50 MeV (number of channels set to 10), while performing absolute electron-proton discrimination for protons up to 200 MeV. In its baseline design, the 3DEES has the capability to measure angular distributions at 12 angles spanning over 180° in two planes. However, for the in-orbit technology demonstration on board PROBA-3, 3DEES is equipped with only one detector head consisting of 6 apertures in one plane. Each aperture has a field of view of ~15°, and the looking directions of any two apertures are separated by 30°. The satellite was launched on 5th December 2024 into a highly elliptical orbit: 60,530 km apogee, 600 km perigee, 59° inclination, 19.5 h orbital period. With these orbital parameters, the satellite will cover parts of the inner belt, outer belt, and mostly the border of the magnetosphere. At the time of writing, the commissioning phase of the spacecraft is ending. This paper presents a description of the 3DEES design and demonstrates the importance of extensive Geant4-based simulations to support the design and characterization of the instrument. To assess the performance of the instrument in orbit, the instrument’s response functions are folded with differential energy spectra obtained from radiation belt models. In addition, the calibration of the instrument in a proton beam, as well as its verification with a 90Sr/90Y source, is presented. Those measurements validate the Geant4 simulations and verify the expected functioning of the instrument.