why we do measure vibration in three direction?

Taking readings in three directions gives more information that helps in analysis as some defects comes with predominant vibrations in a particular directions though not in all cases.

However, it is not necessary to install online vibration monitoring probes in all three directions. In most cases vibration probes are mounted in the radial direction (ie vertical or horizontal) especially in the horizontal direction. When there is significant increase in vibration in any of the three directions, there is a corresponding increase in the other two. By this the online monitoring probes picks up the increase and detailed analysis can then be carried out.

Are you using proximate probe? if so 90 degree apart at verticle direction two probes are to be mounted otherwise if the probe is screwed on the bearing housing you can put another probe at horizontal direction also so that you can monitor the condition more accurately but if the brg housing is cantilever type vertical direction is sufficient.

Some defects and configurations show more in one direction than another.

H&V together allow generation of orbits.

A shows defects that H&V will not.

What else?

thanks!!
In our pumps, only one probe is installed that is in verticle direction by vendor. Pump is multistage and verticaly grouted so always there is low vibrations in vertical direction. We usually measure vibration with TK-80 also with regular interval, what we notice there is always more vibration comparatively in hor. direction. As you said correctly that there may vibration in different direction due to different reasons/defects. Now I want to install one more vibration probe, to justify it what are reference in ISO standard there is recommendation(RP) that is one refence. Please direct me.

Because vibration analysis is alot about trending I was always lead to believe that if the vibration was to increase in one direction it increase in all 3, hence it was a bit of overkill to use 3 sensors.
I am interest to know what faults would you find from an A mounted sensors