You need their other forms from the Dream Radar. Then you need to go to Abundant Shrine. There you will receive the Reveal Glass. You can switch their forms from their on.
For ORAS, once you have Landorus, go to Mauville City and talk to the lady in the mirror shop. She will give you the Reveal Glass which you use to change their formes.
How to Catch The Trio in ORAS
In Pokémon Sun and Moon, to obtain the Reveal Glass, you must travel to the Dimensional Research Lab after you have beaten Olivia and Proffesor Burnet will give you the Reveal Glass