You can only get Xerneas in Pokémon X and Yveltal in Pokémon Y.
However, Zygarde can be caught in both games, which is the remaining Pokémon of the trio.
In terms of Legendaries, most of the ends of the caves in Pokemon X and Y (Like the Sea Spirit's Den in the Azure Bay near Korrina's town
In Pokemon X and Y, Lotad can learn both Water Gun and Razor Leaf if you are willing to lock in the abilities. You have to use HMs to teach both.
Yes You Can Get A Pikachu In Pokemon X and Y.
The Legendary Pokemon available in Pokemon X/Y are:Xerneas (X only)Yveltal (Y only)ArticunoZapdosMoltresMewtwoZygarde
Pokemon X and Y are sold at MSRP of $39.99 (CAD)
If you mean with the 3 legendaries, the legendary bird pokémon Articuno, Zapdos and Moltres: you can't catch them in Diamond. You can migrate or trade them, since they will be roaming Sinnoh in Platinum. You can also find them in FireRed/LeafGreen, HeartGold/SoulSilver and X/Y.
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Prof. Sycamore will give you the oval charm in Pokemon X and Y.
Pokemon capable of learning Psywave as an Egg Move in Pokemon X and Y are Latios (Pokemon) and Gastly (Pokemon).
Yes, they can both be zero.
I believe X is stronger, but Y looks cooler.
*Mew is not catchable in Pokemon x and y, but you can get him online from wondertrades and such. Mewtwo however, is catchable and you can find him in the cave in pokemon village.