We establish a phase diagram of a model in which scalar waves are scattered by resonant point scatterers pinned at random positions in the free three-dimensional (3D) space. A transition to Anderson localization takes place in a narrow frequency band near the resonance frequency provided that the number density of scatterers $rho$ exceeds a critical value $rho_c simeq 0.08 k_0^{3}$, where $k_0$ is the wave number in the free space. The localization condition $rho > rho_c$ can be rewritten as $k_0 ell_0 < 1$, where $ell_0$ is the on-resonance mean free path in the independent-scattering approximation. At mobility edges, the decay of the average amplitude of a monochromatic plane wave is not purely exponential and the growth of its phase is nonlinear with the propagation distance. This makes it impossible to define the mean free path $ell$ and the effective wave number $k$ in a usual way. If the latter are defined as an effective decay length of the intensity and an effective growth rate of the phase of the average wave field, the Ioffe-Regel parameter $(kell)_c$ at the mobility edges can be calculated and takes values from 0.3 to 1.2 depending on $rho$. Thus, the Ioffe-Regel criterion of localization $kell < (kell)_c = mathrm{const} sim 1$ is valid only qualitatively and cannot be used as a quantitative condition of Anderson localization in 3D.
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