Why wave trap in switchyard either in one phase or in two phases why you not use wave trap in three phases?

Most of the time I see wavetraps on single phases. A wave trap is simply a low pass or band block filter. It is used to block the transmission of carrier waves from propogating beyond where they are desired. Usually these carrier waves are injected onto a single phase for each application, but there is nothing stopping someone from injecting on two or all three, other than the additional cost of injection equipment (additional cost that does not provide much benefit, I might add).

The coupling in three phase lines will cause some of the signal injected on one phase to show up on the other two, but it is usually not enough to justify wave traps on these other phases.

So the answer is it isn't needed.