Beldum, Metang, and Metagross are not available by normal means in Pokemon X & Y none of the Metagross line can be found in tall grass, caverns, ect... rather there's a feature of the game that can only be accessed in the post-game that unlocks Metang for capture.
The Friend Safari Zone is a feature found in X & Y that rewards players for having a large number of people on their friends list. The Friend Safari Zone scans your 3DS friends list and registers all of your friends and assigns them a type. It then creates an individualized Safari Zone containing Pokemon of that type. More Pokemon can be found in the Safari Zone depending on if that friend is online, or even playing X & Y.
To find Metang you need a friend with a Steel Type safari zone. Metang is found at level 30 and can have its hidden ability like all Pokemon in the Friend Safari Zone.