Unbreaking 3 isn't optional... You can add it if you want to, along with the half dozen other enchants. Smite, bane, fire, ect ect ect, it's all optional. But sure, most people do add unbreaking III, but the sign sells it to you with Eff V and silk touch.
All these posts above me say two things -when you buy a melon sword, it only has eff 5 and silt touch -most users add ub 3 via anvil/e-book, although you can add any other regular sword enchant if you like
