Scizor is great at picking off weaker targets. Neither Steelix nor Aggron are "weaker" targets. Scizor is basically irrelevant.
If these are random pokemon, probably Steelix. It has better defense, and can capitalize on Aggron's weaknesses better.
If these pokemon are competitively bred, I lean Aggron. Aggron becomes better able to fight back against Steelix by virtue of access to more useful moves. Scizor gains a niche in stalling out the other two with Roost, but if either of them decide to focus down Sizor, its over.
If bred specifically for this fight, Aggron, no question. Magnet Rise lets it ignore Earthquake, Fire Punch/Flamethrower melts Scizor, and Aggron can use that same Flamethrower to capitalize on Steelix' godawful Special Defense.
If throwing in the other two, Lucario probably won't win, actually. It would certainly get the first kill, but that would mark it as the most dangerous, and thus the survivors would focus it down. Scizor actually pulls ahead in the battle royale, since it can roost off in a corner, away from the chaos, and either Bullet Punch or Pursuit to ensure the last 1-on-1 is against the weakest possible opponent.
