Misfire on coil 3

2011 Volkswagen Polo 1.2 Petrol
Car: Volkswagen Polo
Year: 2011
Variant: 1.2 Petrol
Categories: Running Rough, Starting & Power Loss
Hi I will apologise in advance cause I know nothing about cars but would really appreciate some advice.
My problems started about 6 months ago when sitting idol in traffic with or without my foot on the clutch my car would shake, as soon as I pull away the car is a nice smooth ride like normal an no power loss what so ever.
Typically I thought nothing of this until about 2 months ago my engine management light came on, I took it to my local garage who told me I had a misfire on coil 3. This was changed and they put a fuel additive in my tank also and hoped that would solve the problem.
The shaking was still there and after about 24 hours the light was back, I took it back to the garage and it still noted a problem with coil 3 and now also a fuel injector which didn't come up as an issue before. at this point the garage said the cheapest thing to try first is to replace the other 2 coils and see if that solves the problem otherwise we might have to look at fuel injectors which can be expensive.
So finally my question is do you think that is the problem because the garage still aren't sure and I don't have the money to keep trying things not knowing if its going to work. I have tried researching the cost and I am getting prices from £40 (very happy with) to £150 (which I really cant afford 3 of!) but in all honesty I have no idea what I am looking for and I just don't want to hand over money I don't really have and still have the same problem.
Any advice would be much appreciated and sorry for the essay.

Thanks in advance :)

Emma
Problem added: Aug 2, 2016 (7 years ago)
Swap the coils and see if that changes cyl . not firing .If not that check the plug and , if needed , cyl . compression .I had a friend with VW and they said it was a coil pack which the replaced .On picking it up , still the same and brought car to me .On checking it you could hear spark jumping on cyl .Returned to " garage " where they found they had cracked plug porcelain insulator when removing coil pack !! New plug , fixed .If still not right , get it scanned with the likes of Vag com rather than guess and fit parts maybe not needed .
Answered Aug 2, 2016 (7 years ago)
Given the rather random work and "GUESS" work carried out by your garage I'd suggest the first thing after the previous respondents suggestion would be to get a compression leakage test done on cyl #3.
This is just a test to establish "IF" number 3 cylinder is holding compression.
There is a known fault with VW valves and seats.
Just throwing cash at it seldom if ever hits lucky.
And in my opinion just bad workmanship.
Answered Aug 2, 2016 (7 years ago)
Thank you for your advice I have looked on my paper work and the spark plugs have been changed, I arrange a compression test and see what outcome I have from that. Thank you for your advice.
Answered Aug 3, 2016 (7 years ago)
After advise I have had my car looked at again and they have been very thorough, the following has been checked and works fine
Spark plugs
all 3 coil packs
wiring
compression test

I have been advised by the garage to go to VW as it could be an fault with the ECU which I have been told is very hard to diagnose and harder to fix.
Does anyone know of anything else that would be worth a try? I am very reluctant to go to VW as they have tried to rip me off to the tune of £600 before for something that didn't need doing.

Thanks
Answered Aug 16, 2016 (7 years ago)

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