You cant do /enchant unbreaking 3 on an axe

Can't reproduce with a normal axe.
Did your axe already have an unbreaking enchantment? It seems to me like /enchant is not able to increase levels of tools when the same enchatment already exists on it, even if the existing one is lower.

If you do try to enchant the axe again once you've already applied unbreaking, it gives you this error: http://prntscr.com/q0z41s