We can use single-axis meter to do the tri-axis measurement.

The single-axis model can detect signals from one direction at a time and tri-axis meter can detect x-y-z direction at the same time.

So, how to use a single-axis meter to do tri-axis measurement?
Very simple, just rotate or move the single-axis meter around in three directions, x-axis, y-axis and z-axis, and take down the measurement.

What are the advantage of a single-axis meter as compare to a tri-axis meter?

For single-axis meter, you can rotate the meter around to find the maximum signal level detected and at the same time you got the direction of the signal.  The tri-axis detector can not find the maximum signal direction since it detect the signal at all directions.  If you just wanted to know the EMF signal level at the point of detection, then the tri-axis meter can do the job, but you can not find the direction of the signal.  To able to find the direction of the signal coming, the single-axis meter will be a more useful and accurate tool.

For application such as wireless bug detection, WiFi LAN setup, Mobile Phone base station radiation detection, EMF radiation safety prevention, RF leakage detection, etc.  You need to find out where the signal comes from which direction, in order to solve the problems.  This is why most of the meters are single-axis meter.  They are more useful than just measure the signal level only, but also the signal incoming direction to resolve the problems.

There is a RF (Radio Frequency) meter in the market which comes with moving histogram graph display, and it is a useful tool for finding the maximum signal levels and the RF signal directions.  Just move the meter around and it will record the signal levels changes, and it will tell you the maximum signal level around the area and the direction of the signal.  There is also a dual mode meter to measure both RF and LF (Low Frequency).