Is it permissible to move three graves from a plot of land that was purchased to build a mosque on it, after the owners of the graves agree to that?

It is not permissible to exhume graves and relocate their contents for the purpose of building a mosque, whether the landowners agree or not. This is due to the sanctity of the deceased if any remains are left. However, if no remains are left, then there is no harm in exhuming them and building the mosque.