So I've been playing around with Y and after 50 wins on the free battle feature, I think it is safe to say that this team of 3 is not only solid, but overpowered.
Lead
Staraptor
Ability: Intimidate
Nature: beneficial to attack, but not a hinder to speed ( adamant is ideal)
Moves: Brave Bird, Close Combat, U-turn, and whirlwind
Item: Choice Scarf
Staraptor is an amazing lead and scout as he can OHKO almost anything that doesn't resist and put massive dents into almost anything. The draw back is his defense, but U-turn t allows you to flip your role and revenge kill most things. Whirlwind has only 1 perpose, stop Xerneus from setting up. This deer can use some move that raises speed, spec.defense and spec.attack by two stages. Be very careful when facing this monster.
Second
Blaziken
Ability: Speed Boost
Nature: anything really works, but any attack and speed hindering natures just suck.
Moves: Protect, Hi-jump kick, swords dance, flair blitz
Item: Focus sash
"Ummmm Money_man blaziken is uber you noob" Well I say shut up to you. I understand he is very OP, but there is no meta game for X and Y yet and seeing how popular legendarys are, the tier rules don't apply. The game is still in the "Wild West" mode and the meta game is still being developed. But you know the drill with Blaziken. Stall out as long as you can to build speed boost and get at least one swords dance up. You can still sweep away almost anything, even without an attacking nature.
Third
Gardevoir
Item: Gardevoirite
Nature: Modest
Moves: Fairy Moon, Psychic, Calm Mind, and Dazzling Gleam
Ability: Synchronize
So Gardevoir is in my opinion the most powerful of all the mega pokemon. Yes you heard me right, Garchomp is a mere mud fish and T-tar is a little lizard. When in mega, the spec attack is at an insane 200+ with the modest nature ( and decent IVs) and it's speed is high enough to outspeed almost anything that isn't scarfed. With the new fairy typing, you get immunity to Dragon moves, and dark attacks will not OHKO all the time. Calm mind not only raises your spec attack to insane levels, but you can now wall many psychics. I have a feeling that Gardevoir will be in the uber tier in the future.
So how do you get the items needed to build this team. Well after the E-4 you can go to the battle mansion and earn BP to buy the scarf and sash. The Gardevoirite is given to you when you trade with Diantha in Lumiose city. I remember she is in a cafe' so check them all.
Well that is that, a good team with solid coverage for a new pokemon game. With that I give you a farewell and as always give me some feedback and make some inputs if you want to.