Called 3D because you actually blast, or carve, deep into the glass, giving a three dimensional etching. This way, elements can be carved to different depths than the other elements they touch, yielding a 3-D bas relief carving. But not a regular bas relief, a reverse bas relief - since you are carving from one side of the glass and viewing the finished carving from the opposite (smooth) side! In other words, you are carving a hollow (or negative) shape in the glass that looks solid and positive from the smooth side.
