Smiting Blow & VV are a given, 3rd Atma should be based on whatever you're doing and what weapon you have.

Razed Ruins can actually be great for Masamune SAM since the aftermath applies to critical hits which means if you get a double damage proc on a crit you can easily smack mobs for 700+ a swing. But otherwise I like Sundering Slash for the extra Regain + Attack.

The main reason for Suneater is to help enable a 4-hit build btw. I believe it's possible to do without it, but you have to have top-notch STP gear and eat carbonara. If you can manage 4-hit without Atma with a Masamune (it's possible) then you should definitely use either Razed Ruins or Griffon Claw (WS DMG +20%!) for your 3rd Atma.