The Golems Legendary Trio kept closely to the trend of the previous two generations of releasing a Legendary Trio. The three Regis (Regice, Regirock, and Registeel) are based off the Golem Legends of the Hebrew Culture, and they are among the most complex, complicated in-game Pokemon to find and capture.
In order to find each of these Pokemon in the Hoenn Region, a player had to fulfill a number of tasks and then follow steps in braille (yes, braille) in order to find the exclusive Pokemon. It really didn't make much sense at the time of the Games; however, all was made a little more clear when their master, Regigigas was released in the fourth generation.
Apparently, the story goes that the Regis were made in the image of Regigigas by itself, and the people locked all four away in hope that someone would one day come and tame the three creatures to, in turn, control their all powerful master.
Registeel was made from Magma, and specializes equally in both Defense and Special Defense; Regirock was created from clay, and specializes a little more in Defense than Special Defense; and Regice was created from ice, and specializes a little more in Special Defense than Defense. All three Pokemon are cleary used for their Defensive Powers.
#377 - Regirock: The first of the Regi Trio can be found in the Desert Ruins after completing all necessary tasks from the braille script, and for all three third generation Games (Ruby, Sapphire, Emerald) and their remakes (Omega Ruby & Alpha Sapphire). It can also be found in Pokemon Platinum in the Desert Ruins on Route 228, and in Pokemon Black 2 and White 2 in the Underground Ruins.
#378 - Regice: The next Regi can be found in the same exact Games: the Island Cave on Route 105 in the third generation games and their remakes; the Iceberg Ruins in Mt. Coronet in Platinum; as well as the Underground Ruins in Pokemon Black 2 and White 2.
#379 - Registeel: The same thing can be said for Registeel, the Games it is found and such: the Anicent Tomb on Route 120 in Ruby, Sapphire, Emerald, Omega Ruby & Alpha Sapphire; the Iron Ruins on Iron Island in Platinum; and finally, like the other two Regis, the Underground Ruins in the fifth genration sequels.