The Unbreaking III enchantment works as if it was Unbreaking I.

When using a fully enchanted diamond Pick, Unbreaking III will take 1 - 2 uses to go down in durability. Unlike the 2 - 3 uses it supposed to be.

Reproduce:
Enchant a diamond pickaxe with Unbreaking III and mine the appropriate blocks. You will get the bugged result.