What are three main causes of sever boredom and how to fix?

You can look up fun games to play solo or that are easy to make with common house items (since you're grounded). Or you can try origami, find some new games to play online, find a new youtuber to watch, if you're into reading then find something new to read on Wattpad/any other online ebook source---the options are endless.

No TV, No phone, and no book. Fix it by going outside and playing something. Plant stuff, garden, call up friends and see if they wanna hang out.