3D pixel sensors are a novel type of silicon detector designed for high energy physics and other applications. Their features include high spatial resolution and radiation tolerance. The design consists of alternating rows of n- and p-type columns in a p-type substrate. Capacitance data are presented for 3D pixel devices characterized after exposure to various fluences of 55 MeV protons. Simulations and two methods for the measurement of the inter-electrode capacitance as a function of input signal frequency are shown.