The montecarlo3d command computes an approximate volume for the the region where f(x,y,z) is true inside x0 <= x <= x1, y0 <= y <= y1, and z0 <= z <= z1.
montecarlo3d(x^2+y^2+z^2 < 1,x,y,z,-1,1,-1,1,-1,1); 4.18914365768433