Some scans are done in 2D, some are done in 3D. Sometimes on a 2D scan, how big the shortest measurement is (the "short axis") is considered the most relevant -- such as sometimes with an enlarged lymph node.
On a 3D scan, you can multiply all 3 together to get the volume. So 2*3*2 cm would give 12 cubic centimeters.
On a 2D scan, you can only get the area instead of the volume. But using an enlarged lymph node example again, the ratio of the length over the width (L/W) is much more relevant than the area (L*W) because the shape is important.
I do not know because my mass ultrasound report listed the 3 measurements. I will say that knowing the 3 measurements tells us more, whereas volume alone doesn't seem specific enough to me, unless the mass is round, so then volume makes more sense. Patients tend to say whatever the biggest number is to their friends/relatives.
Mine looked like a skinny Crunchy Cheeto stretched out horizontally across the ultrasound imaging video, so my last number (7) was the biggest number. I have heard that wide is better than tall but don't know how true that is, and really it all depends on what angle you are viewing something from.