how to hear plug ins in sonar x3 producer

It is a combination of interface and DAW config. To hear effects while recording, the recording track(s) input echo button(s) must be enabled in SONAR and the interface direct monitoring should be disabled. This will cause all audio to come from the DAW. It also means you will have to deal with the round trip latency caused by monitoring through DAW. Depending on the driver mode in SONAR the latency will need to set as low as possible in Preferences > Driver Settings or the ASIO client supplied by the interface vendor.

Just a little background...all DAWs record the dry guitar on a track, and you hear the amp sim processing the dry track during playback. This is why you have to enable input echo; it sends the guitar into the computer, where the plug-in thinks it's a dry track, and processes it.
 
As scook points out, there is some delay when monitoring through the computer. This is caused by sample buffers within the computer, buffers on the USB port (if you're using a USB interface), and A/D/A conversion. Also, plug-ins with "lookahead" (like some limiters) causes a delay for which SONAR must compensate by delaying all audio.
 
I assume you are using an external interface which is capable of using an ASIO driver. With a modern computer (and interface with a well-written driver), that should allow a "round-trip" latency of under 10 ms. If you have to use a driver like MME with an internal sound card, the delay will be so long it will take all the fun out of playing.
 
Please check out this article in the Cakewalk blog if you want to know more about latency basics.

It is my understanding that SONAR's Help covers this and a whole lot more.  I also believe that Cakewalk makes the manual available on their website.  I remember someone had posted a protocol to follow before posting questions.  Of course there are many here who want and willingly go out of their way to help, but issues like this are easily resolved when any individual references the readily available information in SONAR.  Just hit the F1 key for a start.
 
Okay, rant over.
 
Kind regards,
 
Living Room Rocker