does anybody know how to use rxp in sonar x3. will thankfull to you.

These videos are dated, but they still apply.

 
You can also Load Rex files in to RXP, edit them, then drag them from RXP on to the timeline (in to a track)
its basically a REX file editor/VST-Instrument similar to PropellerHeads Recycle (minus a lot of the options Recycle has).
 
We are talking about a dated piece of software (Sonar 5 I believe)
http://www.soundonsound.com/techniques/using-rxp-rex-file-player-sonar-5-producer-edition
But it still has its uses. I just assume split beats in the timeline and manipulate them there, But RXP can do some of that easier.