The 2-D CAP (Carrierless Amplitude and Phase) modulation system has been described in several ATM Forum documents as a viable modulation scheme for realizing very high bit rates on UTP-3 copper loops. Recently, Shalash and Parhi contributed a technique to extend the CAP modulation scheme to 3-D. This paper investigates the properties of the adaptive 3-D CAP digital receiver for VDSL in the face of quantization noise effects due to the ubiquitous ADC circuitry in the receiver. Our simulation results show that the original 3-D CAP receiver is fairly sensitive to the quantization noise. We will explain why and then contribute a solution which involves a re-design of the signature waveforms that offers much better reception properties at the 3-D CAP receiver.