The spectrum of collective intersubband magneto excitations in a quasi-two-dimensional electron system confined in a single quantum well has been studied by inelastic light scattering spectroscopy. Intersubband Bernstein modes with a change of the Landau level index up to three were observed in a perpendicular magnetic field. It is demonstrated that Bernstein modes couple with intersubband charge and spin density excitations through many-body Coulomb interaction, and the coupling strength is governed by the in-plane momentum. Detailed measurements of the coupling strength dispersion were performed and compared with theoretical simulations.