We present high-resolution measurements of the isothermal susceptibility of pure 3He near the liquid-gas critical point. PVT measurements were performed in the single-phase region over the reduced temperature range 3 x 10(-5)<T/Tc-1<1.5 x 10(-1). The crossover behavior of the susceptibility along the critical isochore was analyzed using a field-theoretical renormalization-group calculation based on the phi4 model. A similar crossover analysis was performed on previously obtained Xe susceptibility measurements. A comparison of the rescaled susceptibility for 3He and Xe shows theoretically predicted universal crossover behavior.