Why did #3 cylinder spark plug striped when trying to remove it

It maybe the plug was cross threaded when installed. It is’nt the end of the world though. You can buy a rethreading kit for spark plugs these days, fairly inexpensive too. I had to do the same job on my motorbike engine. I got a 15mm Helicoil kit that came with a 16mm thread tap. Once id run the tap down the hole of the alloy head, the helicoil then screwed in & you would never know there had been a problem.

Heres a similar kit to the one i used. I know my motorbike is different to your car engine but it is a similar process to repair the stripped thread.

https://www.amazon.co.uk/Retrofit-Keenso...