Up to W7/EL, I always viewed Luffy and Zoro as close to equal as equal goes (i.e. a fight between them would have a similar conclusion to Dorry's and Broggy's frequent clashes) with Sanji a small, but somewhat noticeable, distance behind them. I inferred thus given the, in my opinion, sense of equality Oda has portrayed between Luffy and Zoro ever since Zoro joined Luffy's merry-band of pirates.
With the introduction of Gear Second however, I felt that Luffy pulled away from Zoro. I'd say that they were still roughly equal in "base", but that in Gear Second, Luffy was a step above Zoro since Luffy paid a price that Zoro did not: he literally sacrificed years of his life, pushing his body to his absolute limits and then some, in order to achieve that level of combat prowess.
After the time skip Luffy's Gear Second seems different though; he no longer seems to extract a very heavy toll for using it, and it now seems to be more of a fundamental part of his fighting style, rather than a triumph card (i.e. I now view Luffy's Gear Second (and likewise, Sanji's Diable Jambe) similarly to Zoro swords). In addition, the way Luffy and Zoro simultaneously shut down Decken and Jones, respectively, gave me the wibe that the gap between G2 Luffy and Zoro is no longer as evident as it was (imo) before the time skip.
That being said, any fight between two of the Monster Trio would be a very tough and intense fight, but I think the end result is given in such a competition: Luffy defeats Zoro and Sani, and Zoro beats Sanji.
Zoro may wield swords, which some would argue is a superior fighting style to Rubber Kung Fu or Fire Leg Karate and that is perfectly true in our world. But One Piece is a different verse, and a fictional shounen verse at that, and I think past chapters (i.e. swordless Kaku vs. Zoro, Luffy vs. Zoro in whiskey peak) have proven that weapon styles doesn't have any significant advantage against hand-to-hand styles.
Regarding the free for all scenario, I actually think Sanji has the biggest shot at winning, if they are in character. Sanji is a more calculating and tactical combatant than the reckless Luffy and Zoro, and I think Sanji is more like to play it safe in the beginning while taking advantage of any opening to damage either of his opponent. In the end though, the winner could be either one of them, and would be decided by circumstantial luck (i.e. the third party intervenes at exactly the right moment).
EDIT: One a side note, I firmly believe that Zoro and Sanji has learned the CoA and CoO forms of Haki during the time skip, considering that it took an alleged Haki-genius like Luffy one and a half year of hardcore training from the Dark King to learn it, and I can't really see Zoro or Sanji, the two most powerful combatants of the future Pirate King's crew, growing into top tiers without it - nor can I imagine Luffy spending the next 2-4 years training Zoro and Sanji in Haki.
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