Well,here's a tip for encounter the beasts at the first time.
1.go to the burned tower deep where you scare the three.
2.for entei,go to the route south from ecutreak,west for suicune,and east for raikou(near mahogany(surf))
3.every time you encounter or catch one and you want to change target,do this all over again.
If you want to get it quick,use repels on the routes with level 40 or lower pokemon in your party.i recommend butterfree but don't miss the sleep powder(15)and give it quick claw,and use sleep powder then just lower it's hp.every time you will use moves or balls,suspend.i caught all in three hours but can't guarantee for you.