It is not possible for me to speculate if National Instruments will be developing a 3.3 V MXI-3 board, but we currently do not make one. We also do not have a solution for converting a 5 V signal from a PXI backplane to 3.3 V for a PXI card to use. I would recommend checking www.picmg.org for other possible solutions.
Ryan Tamblin
Applications Engineer
National Instruments