To start off, all 3 of your Pokemon have a very similar EV distribution, so you're probably thinking about which of the 3 sweepers would be the best.
Gardevoir's nature works great with your moveset and your EV distributions. The ability Trace is sort of a moderate ability because you don't know what to expect from it, and if it will benefit you in any way. Synchronize may work slightly better in this case; but definitely Telepathy is wanted for doing doubles in the battle tree - this providing you don't choose Mega Gardevoir. However for Mega Gardevoir, if you're doing singles, you would probably want to have a move like Moonblast > Hyper Voice, as it is a little stronger, but keep Hyper Voice if considering doubles. For Psyshock, it's sort of an unknown case when deciding if Psychic or Psyshock will work better for you, but I'd still consider taking Psychic > Psyshock. Your other moves are fairly good, and you probably included Will-O-Wisp to benefit Gardevoir's low defense stat. For Focus Blast, it does help take care of its steel-weakness, but you may find it easier to take out its Ghost-weakness with Shadow Ball instead of Focus Blast, as steel-type Pokemon usually have a lot of defense.
Charizard's nature also works well with your moveset and EV distributions. Since Charizard has two mega evolution, and your Charizard has Solar Beam, it would make a lot of sense to choose Mega Charizard Y - and you also don't have any Dragon-STAB for Mega Charizard X. For singles, you probably pick a move like Fire Blast > Heat Wave; but Heat Wave seems like a much better STAB move in both cases, particularly doubles. Your other moves are good, but you may consider having Sunny Day to benefit the ability if you don't want to mega evolve Charizard
For Blaziken, it's nature is not as good as the others, as any ability that usually decreases a defensive stat is not worth it - also knowing the Blaziken has moderately low defenses as well. Both Blaziken and its mega evolution have the same typing and ability, so there's now concern to that - it's just the boost in stats. Blaziken's attacking stat is also not as high as the other Pokemon, so you'll need to rely on speed; which is why starting off with Protect and having it's speed increase is the best choice. STAB move for its Fire-typing is good, but I think a move like Sky Uppercut or Brick Break is much better for its fighting-typing. Blaziken also seems like a Pokemon that can work for singles or doubles, so it's optional for you on how to use it.
Overall, I think all Pokemon are good, but I think you may have the best chances with your Mega Gardevoir or Mega Charizard Y. Since you are only looking for one, I would recommend looking at your IV's for both Pokemon and make a decision based off of that. If you're unable to, I think Mega Charizard Y would be my recommendation; and you probably don't want to have 2 regular fire-type starters on your team.
However, if you do think Mega Gardevoir is your choice, I would recommend giving a Heat Rock to Charizard to benefit it's ability, but you would need to either learn Sunny Day or have an ally learn it (with the exception of the ability Drought). For Blaziken, maybe have it hold either an Assault Vest to counterbalance with it's hindering nature for SpDef, or maybe an Expert Belt if you want a really strong sweeper. Otherwise, if you take my suggestion with Mega Charizard Y, maybe give Gardevoir a Sitrus Berry or another supereffective-weakening berry.
Your Choice! Hope I helped! :)