Legendaries will not be catchable per say, but will be unleashed in events one at a time. Trainers may have a chance at catching them only 3 times. If every trainer fails 3 times, then that legendary is gone until the admins decide to bring them back for another round.
The exceptions are:
Arceus
Darkrai
Ho-Oh
Dialga
Palkia
Giratina
Lugia
Mew
Deoxys
Jirachi
Celebi
The above legendaries will NOT be released in an event unless otherwise stated.