This paper reports an attainable channel capacity and spatial degrees-of-freedom of multiple-antenna radio channels at 15, 28 and 61 GHz. The analysis is based on channel sounding in a same street canyon. The attainable capacity is evaluated so that it depends only on the antenna aperture size but is independent of implementation of antenna elements on the aperture. The analysis shows that multipath richness decreases as the frequency increases, as indicated by the smaller spatial degrees-of-freedom at higher frequencies for the same electrical aperture size of the receive antenna. The analysis furthermore reveals that channels at the three radio frequencies can attain almost the same level of capacity for a given transmit power and physical size of the receive antenna. The result is explained by a greater electrical size of the antenna aperture at the higher frequencies that can leverage higher gains and finer angular resolution and hence normalize the availability of fewer multipaths.