All three bring completely different things to the table.
Walrein is a bulky water, dealing a lot of damage with it's large special attack stat. It has a small movepool, but ice beam, surf, waterfall and body slam would be a good set.
Flygon is a fairy strong mixed attacker. It has a large movepool, with things like fly, strength, crunch and flamethrower, just to name a few .It also has an immunity to ground, because of levitate. Just slap on two stab moves, and mix and match the last slots with what you need.
Absol hits like a truck, but is fairy slow. It has a very wide special movepool, and its special attack isn't baaaad so they're viable. Sadly, it has a small physical movepool this gen, and can really only use shadow ball.
Due to you lack of a water HM user, Walrein is a must have. However, you still have options like Sharpedo or Gyarados on that front. Flygon's levitate is very useful for Manetric, so I'd also use that. However, Absol is also a decent.