I can juggle 3 balls. I don’t necessarily think it’s good for focus, however. @Kropotkin is correct that meditation is an actual exercise that can strengthen focus.
What I found fascinating about learning to juggle is that you almost can’t focus on it all. You have to practice until it becomes muscle memory. Shortly after learning, I realized that I could have full conversations with people while juggling. And at no point was I saying, “ok, now I need to catch this ball and throw up the other one”. I wasn’t even really looking. The catches and throws happen at the periphery of vision and consciousness. Turning attention to the motion invariably caused me to fail and drop them all.