Unbreaking 3 not appearing

I have been trying to get unbreaking 3 on my pickaxe and elytra, but when I go to put unbreaking books on them in an anvil, it charges me levels and stays at unbreaking 2. I tried to put 1 unbreaking 3 on my elytra, but only unbreaking 2 was on it. For reference, I am playing on a realm.