We investigate the presence of plasma resonances in InGaAs n+−n−n+ diodes under different optical excitation conditions. In particular, we study the case of diodes submitted to an optical photoexcitation presenting a beating in the terahertz frequency domain. For this purpose, we calculate the electric field response in the middle of the n and n+ regions using a hydrodynamic approach self-consistently coupled to a one-dimensional Poisson solver. In particular, the analysis of the electric field response to an optical beating as a function of the doping and the geometry of the devices allows us to evidence in all the considered cases the presence of resonances in both n and n+ regions. However, while the observed resonances agree with the theoretical 3D plasma frequency in the n+ external regions, we point out a shift towards higher frequencies in the n region. We show that this shift towards the n+ 3D plasma frequency is due to the strong coupling between the two region modes, and tends to disappear when the n region lengthens, whereas the influence of the n+ regions length on the resonance frequency is negligible. Moreover, we show that the amplitude of the plasma resonances can be enhanced at high doping levels and by increasing the level of the optical photoexcitation. The obtained results show clearly that the resonances associated with 3D plasma waves in InGaAs diodes lie in the THz domain for typical values of dopings and lengths, thus opening new possibilities to exploit not only field effect transistors but also diodes as solid-state terahertz devices operating at room temperature.