There's no built-in function of which I'm aware for calculating volumes in AC3D.
However, I had some code lying around for calculating 3d volumes, so put it together for AC3D.
Unfortunately, it only works for the Windows version, but the source code is available.
http://supercoldmilk.com/ac3dplug/calcvol.html
Be sure to check out the Exceptions section, as your geometry must follow certain guidelines to get an accurate volume reading.
Let me know if it works for you and I'll post it to the plugins area.
Dennis