The use of a tri-axial accelerometer is for:
1) Space restictions. Tri-axials tend to be smaller than 3 single axis transducers.
2) Mounting considerations (on a flat plain/surface it would be difficult to get the horizontal directions using conventional single axis transducers without using a mounting block.
3) Comfort considerations. This is related to space considerations. A tri-axial accelerometer mounted in a seat pad for human vibration measurments would be smaller and more comfortable to sit on than a block of 3 single axis accelerometers.
4) Measurement location. The vibration signals measured are at the point of attachment of the tri-axial. If 3 separate accelerometers are used, they may be separated by small distances depending on the surfaces which are available for mounting.
5) Depending on the actual equipment, it is easier to set up and connect a tri-axial accelerometer.
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