Definitely keep Scrafty and Swampert. Something to set Trick Room for their lackluster speed stats would be welcome, and Gallade's pretty sweet, so go with that. For a third, I'd say grab a Durant. With those slow Pokemon, something fast, but still hard hitting (Hustle essentially gives it 190 Base Attack) (and it also has very good physical bulk). Or Archeops
Sets I'd recommend would be:
Scrafty with Moxie, if you use Dragon Dance, or Intimidate. HJK, Crunch, Dragon Dance, Ice/Thunder Punch.
Swampert with Torrent. Dive/Waterfall, Earthquake, Avalanche, Curse. Yawn, Double-Edge, Hammer Arm or, if you go back to E12 or earlier, Rock Slide would be good options as well.
Gallade with whatever ability. Leaf Blade, Psycho Cut, Close Combat and Trick Room. With a Scope Lens, LB and PC will have a 50% crit chance. I guess you could also fit Destiny Bond on there somehow if you want it, works well with Trick Room.
Durant with Hustle. Iron Head, Dig/Crunch, Thunder Fang, X-Scissor. If you go back to E12 or earlier Rock Slide and Hone Claws are also available.
Archeops with Defeatist (sadly). Rock Slide, Acrobatics, Dragon Claw, Knock Off/Endeavor. There's also EQ if you go back to E12 or earlier.