I am stuck on a UNIX legaxy project. This is the perfect time to learn awk, sed, vim. I use them but I wanna get better.

I can read a book, course, but I WANT to write code, maybe something like the hackerrank challenges but harder.

How do I get advanced in awk, sed, vim?

There are books about them, filled with every esoteric option available, but the best way to become an expert with them, as with anything, is to use them a lot. There's no shortcut.

For vim, I recommend you to search for stuff like "writing your first vimrc".
It will guide you into making it your truly own editor, no one else will be able to use.

this is not what you asked for (a challenge), but I found these tutorials which seem to be the least eye cancer inducing (clean UI, only 1 ad) or least eon consuming I could find:
https://linuxhint.com/category/awk/
https://linuxhint.com/category/sed/
(I haven't use them myself and are, again, tutorials, not extensive, not what you asked for)

regarding vim, upcase's vim tutorials are the perfect compromise between quality and time economy. They used to be paid but they are free now.
https://thoughtbot.com/upcase/vim
(this one I did use, and thanks to them I use vim everyday)

@bahua Clearly! I read so many things about Vim, but if I'm not using them, you can be sure I'll forget everything.

I only really became skilled by using them.

You start out a begginer, master the basics. And then, you start asking - can this thing that I do a lot, simplified somehow?

And so you learn advanced functions, macros, shortcuts and so on.

@bahua @Aldar I agree, I use them every day, but I am stuck at a certain level because I don't know the tools' capabilities e. g. I don't know what I don't know