When beat the Elite Four, Professor Oak will come to congratulate you for your achievement. Then he will leave. He will be at the Pal Park. To enter the PalPark, you must have the National Dex. Then, when you enter to the building, Professor Oak will come to your place and talk to you. Before he finish he will mention the Eterna City. After that he will leave. Go to Eterna city. Professor Oak lives in a house which is located at South. Talk to him and he will give you a poketch app. Then he will tell you that 3 legendary pokemon from Kanto lives now here (Sinnoh). In your poketch app you will be able to locate the 3 legendary pokemon, in the same way, as you did to find Mesprit.
I hope that i have help you.