Mega Venusaur has an increase in defense stat and special defense stat. It also gains the ability "Thick Fat" meaning it halves the power of both ice and fire type moves.
Mega Charizard gains a significant growth in the special attack stat and gains the ability of "Drought." Drought will give Charizard an increase in fire attacks and also decreases the attacks of water moves.
Mega Blastoise gains a new ability called "Mega Launcher" which increases the power of pulse moves such as water pulse and dark pulse.
To learned more about Pokemon X and Pokemon Y as well as the Mega Evolutions for the other pokemon follow the links below: