I would suggest starting with the ignition component of the combustion.
1.) one at a time, while the engine is idling take off a spark lead from your coil pack. if it idles more roughly it would suggest that it is not that cylinder, replace the lead and check that the idle returns to how it was prior to removing the lead.
2.) repeat this for each lead, if removing one or more of the leads does nothing to effect the engine's idle it would suggest that there is an issue with those leads/spark plugs of the coil for that cylinder.
3.) if none of the leads makes any difference it suggests that your plugs, coils & leads are ok, you may be looking at a fuel or compression issue, i.e. an injector may be blocked or require cleaning or you may have fried a piston ring. if this is the case a professional may be required.
hope that helps
Just a note: don't use your bare hands to remove the leads, it will shock you. try carefully using a set of good pliers to gentle pull off the lead.
Don't take a spark lead off while the motor is running, you may get a shock and damage the module, to test if a cylinder is not running disconnect one injector plug at a time and listen if the revs drop and idle diminishes.
This way you also keep un burned fuel out of the exhaust/cat.
Ok i checked the leads and the injectors on the weekend now I did the process u told me for the injectors BUT yes one failed NO#3 when i pulled it off while engine was running it died straight on me but when i pulled all others off the engine revs just changed and the engine didnt die!
So now iv found the problem with injector NO#3 what should i do??