I present the results of 3D general-relativistic magnetohydrodynamic simulations of the differentially rotating neutron stars performed with GR-Athena++. We study the magnetorotational instability and the turbulence it induces by looking at distinct patterns in the magnetic field, and the interaction between this field and the fluid respectively. Additionally, we measure the effective shear viscosity in order to calibrate future simulations, and finally we compare our results with 2D simulations in the literature.