If you have a 3.1 x 2.1 x 2.2 tumor the shape of a rectangular prism, the volume would indeed be 14.3 cm3. However, for a tumor the shape of an ellipsoid, the volume is 4/3*PI*r1*r2*r3. This gives 4/3*3.14159*1.55*1.05*1.1 = 7.50 cm3.
The intrameatal volume is negligible for a tumor this size.
The vestibular schwannoma literature likes to use the approximation of multiplying the 3 diameters together and halving the number. This approximation gives you 7.16 cm3.
(In my case, I estimated the volume by superimposing an ellipse in both the axial and coronal views, ensuring that the appropriate matched dimensions matched. If the shape is irregular, ensure, by eye, the ellipse has roughly the same amount of tumor outside the ellipse as inside. This "more accurate" figure has a very small effect on the calculation and certainly less than the abovementioned approximation sometimes used by the literature. The intrameatal volume was a very small addition to the overall volume.)