Hi All . Up to now I've only used the step sequencer for session drummer 3 but have recently aquired an old roland v drums kit. I've no experience of connecting midi and haven't a clue how to set the v drums and session drummer in order to play the kit using session drummer 3's samples . Please help ??

You need a drum map that translates the MIDI messages from the kit for SD 3
 
I'm not sure if there's one that comes with SD 3 (depends on your v-drums module as well because they are all different). I know Kontakt provides mappings for certain v-drum kits, but even there you might be better off creating your own drum map for flexibility (this is what I did).
 
Best start would be to record a simply kick-snare-hihat groove plus a simple tom break and to see which MIDI message come into Sonar, then pull up the drum maps section of the manual and remap your v-drums groove to SD3. once the simple stuff works, go after the rim sounds, cymbals etc.
 
Make sure you save the drum map and create a backup :-)

Which Roland kit do you have? The more advanced ones, even older models, allow you to easily designate kit pieces to midi channels etc.

It's a Roland, it will be GM. No need to re map at all. 
I would see if you can re set it to factory default in case someone messed with it. 
A lot depends on your drivers and system set up. If it is modern and fast then you might be fine with triggering the SD sounds directly.
 
My old system was underpowered and there was a noticeable latency. If I monitored my Yamaha kit (DTX450)  directly and mixed that with the output from session drummer there was a very noticeable slap echo.
  So I would do my overdubs using the Brain for monitoring. This gave me solid timing. 
Simply turn off input echo on Session drummer while tracking to get rid of the echo. 
My new system is much better so the latency induced echo is  withstand-able but I still record with echo off.
But my Yamaha has pretty good sounds so they don't bother me for recording purposes. But- If the Roland's brain sounds are terrible then ya, you might want to use the Session Drummer sounds for inspirational playing.  
 
As far as hooking it up, You'll be using your audio interfaces midi input. Make sure it is enabled as a midi device in midi preferences. Then insert a midi track and set the input to your midi port and the output to session drummer. 
 

It's the TD - 6V . I was assured the process would be fairly simple as its all roland but the word simple is often misleading on the world of music software .

That's good advice . Was hoping to avoid mapping , like you say its all roland . I'll give that a go . Thanks

Also , will sonar instantly recognise the v drums in midi devices or will I have to do a search or download drivers of some kind ?

I use my TD4 and it is already in Addictive drums. There are lots of youtube videos to help you set it up.