The range of a sonar based sensor is highly dependent on the nature of the target itself. Not only the shape and size of the target, but the composition of that target, since sound reflects differently off different materials. You're only going to see that advertised maximum range against an ideal target, and a soft, sound absorbing, human body (assuming with cloths on, which are even more sound absorbing), is nowhere near an ideal target. (an ideal target being a flat, hard surface that offers maximum sound reflection)
The datasheet itself shows the characteristics of two different targets, a cylinder and a 12x12 piece of cardboard, and neither one reaches the advertised 3m maximum range.