I'm seeing 4-5 different sizes at a glance on my bike.
Even if there were only 3, my point was that you can't rely on a random key you got from Ikea, and you're not always going to run into the same 3 sizes on every bike. Sooner or later you'll have to buy a full set with 6 or 7 sizes, plus a large key for the stand screw (if you have a stand).