Yes. In the Buried Relic dungeon, you face the three regis on floor 15, 25, and 35, one on each floor. The first time through you can't recruit any of them, but after that, you'll just have to keep going through and beating them again until you recruit them.
i think floor 30 or 45
you need to keep trying to deafeat him ( i sugjest using a friend bow )
aegis cave at level 49, along with regirock, regice, and registeel.
To Get the regis in Pokemon Mystery Dungeon You have to beat kyogre ( Stormy sea) later you should get a mail it talks about Ruins then talk to shiftry and you can go to buried relic Regirock 15 floor Regice 25 floor Registeel 35 floor (Mew 36-98 floor)
Buried Relic
As soon as you receive a leter for the discovery of the hidden land, you can enter. At the 15th floor, you can fight Regirock, at floor 25, Regice, and Registeel at floor 35.
NOTE: You can only get this after beating the game. After you beat the dungeon 'Stormy Sea', talk to Lombre, then Shiftry in the Pokemon town square. Shiftry will unlock the dungeon 'Buried Relic', which contains Regirock (Floor B15), Regice (B25), and Registeel (B35). If you beat all three, you get a Music Box, which makes Mew appear anywhere in floors B36-99.
I think you just collect the rock, ice, and steel parts again by defeating regirock, regice, and registeel again.
If you want to recruit the regis, you need the part that you got from the particular Pokemon (like the steel part for registeel) or the music box. After you get one of those, redo the dungeon and keep on beating them until they join your team.
the rock part from regirock, the ice part from regice, and the steel part from registeel combine to create the music box, which i believe is used to recruit mew, but i am not sure.
NOTE: You can only get this after beating the game. After you beat the dungeon 'Stormy Sea', talk to Lombre, then Shiftry in the Pokemon town square. Shiftry will unlock the dungeon 'Buried Relic', which contains Regirock (Floor B15), Regice (B25), and Registeel (B35). If you beat all three, you get a Music Box, which makes Mew appear anywhere in floors B36-98. Hope this helps. ~WREmix
When you complete any random missions after graduating from the guild in Pokemon mystery dungeon, you can go down into the lowest floor of the guild and you will meet team charm, from the special episode. You go with them to Aegis Cave, where you have to gather Unown stones to proceed and fight the bosses Regirock, Regice, Registeel and Regigigas. The bosses only seem to appear the first time you go to the cave, though. From Neonspectre