Engine misfire

Does it only do it once, or can you go someplace, shut the car off and it will do it again when you leave and have to stop?
Does it seem to matter if it is cold or hot outside?
If it starts missing and you just push in the clutch does it shut off or keep running bad?

I asked about the weather/temperature to see if it could be something like moisture or condensation causing a miss.
A test you can try to see what cylinder it is, Disconnect the wire that feeds the coils one at a time when it's running bad. If you find one that doesn't seem to make a difference you have a starting point. On a 3 cyl one lost cylinder is a big issue.
Do you have access to a scan tool that can read live data? Or a shop you trust that has one?

OK now for the fun part. Tracking the reason. Going with the idea that you hate to toss money away we will use a few low cost tests. First try swapping just the spark plug with one from a different cylinder. No change? Try swapping just the coil.
If you still have no change get a test light and verify that you are getting power to the coil feed. Then test for power/ground at the injector.
If moving the various components to other cylinders doesn't change the location of the misfire to a different location IE: moving the #2 plug to #1 and #1 plug into #2 but #2 still misfires. The issue is internal to the engine. For that you will want to run a compression test on all 3 cylinders, and a leak down test if that shows low compression in #2 cylinder. Hopefully it will be a bad feed to the coil or injector.

Please let us know what happens so it will help others.

Cheers, Ken

I would replace that plug not uncommon for a plug to be bad out of the box. Drop them once and the porcelain can crack enough to cause a problem.
Shaky usually means misfires. If you can get the codes read it might point you to the same cylinder or to the one you put the plug in. In the states most of the parts stores will read the codes for free not sure if they do that where you are.

However as you seem to be interested in doing your own work you might consider getting your own DIY type scan tool. There are a few that are $100-150.00 that do more than just read the codes.
This one has pretty good reviews and I know a few folks who have used it and say good things.
https://www.amazon.com/Topdon-Universal-Diagnostic-Professional-Elite/dp/B01MCXXVH1
You can download the manual for it and see if you think it would help you. Plus if you get used to using it you might have friends who you can help if they have problems.