Well the desired Ability you should have for Axew, Fraxure and Haxorus is Mold Breaker, since this grants them the ability to ignore Abilities such as Levitate, Sturdy and Multiscale.
Although Unnerve is not the most pointless Ability, as it nullifies the effects of common berries used in the meta game such as the o so common Lum Berry, used mainly to cure Pokémon of a Burn. As well as Pokémon holding a Sitrus Berry to restore HP or a Chesto Berry complemented with Rest. Axew, Fraxure and Haxorus with Unnerve would be best to use in the VGC.
Hope I helped :)