Aims. We use multi-spacecraft observations of individual type III radio bursts to calculate the directivity of the radio emission. We compare these data to the results of ray-tracing simulations of the radio-wave propagation and probe the plasma properties of the inner heliosphere. Methods. We used ray-tracing simulations of radio-wave propagation with anisotropic scattering on density inhomogeneities to study the directivity of radio emissions. Simultaneous observations of type III radio bursts by four widely separated spacecraft were used to calculate the directivity and position of the radio sources. The shape of the directivity pattern deduced for individual events is compared to the directivity pattern resulting from the ray-tracing simulations. Results. We show that simultaneous observations of type radio III bursts by four different probes provide an opportunity to estimate the radio source positions and the directivity of the radio emission. The shape of the directivity varies from one event to another and it is consistent with anisotropic scattering of the radio waves.