Who’s debuting under the "Big 3" this year?

The most anticipated release from SM Entertainment is their global boy group, EXO (12 members).

The boy group will be divided into two teams; EXO-K and EXO-M. The name EXO is from "EXOPLANET", which is used to express themselves as a star from another planet while "K" and "M" stand for Korean and Mandarin.

YG Entertainment, meanwhile, has two new groups scheduled to debut this year.
Representatives revealed, “Just like the debut of Big Bang and 2NE1, the new groups will take on a color different from their peers.” Their girl group is scheduled to debut in the first half of the year and is comprised of a few known names like ‘Superstar K2‘ contestant Kim Eunbi.
Details on their boy group have yet to be announced, but many are already predicting that they’ll have a music and fashion style that will shake up the industry.

JYP Entertainment plans to focus on the development of their artists globally, as opposed to debuting any new talents. So, Wonder Girls will begin their American promotions, while 2PM focuses on their Japanese promotions and Miss A will centering on their Chinese promotions."Like their other ‘Big 3′ counterparts, JYP Entertainment is also in the process of debuting a new boy group. The members are currently still being discussed, as is the debut date."

 source: Allkpop.