Best way to make an "easy, medium, hard" picker

I'm trying to create a website where you can get questions based on different settings. One setting is difficulty. For now, let's just assume that I have divided a lot of questions based on difficulty: Easy, medium, and hard.

Now, what I want the user to be able to do is to pick whether a certain question should have the difficulty easy, medium, or hard. However, they should be able to choose several. So if they want easy and medium question, they should be able to choose both, or easy or hard for example. So in principle this should be check boxes. My main concern, UX wise, is that "easy, medium, and hard" pickers are often either one of them, so either easy, medium, or hard. Not multiple at the same time. Also, I wish to be able to do this in one line, so it should be multiple lines, and it should of course be accessible for mobile devices as well.

I was thinking of just making three buttons of some sort. But does that really indicate that all three can be pushed at the same time, or only some of them ? Should the buttons be connected, and not be separated. I'm really just wondering how to make the best mobile friendly option of choosing multiple difficulties.