Bagon- Go to Rugged Mountain. You need 10,000 Dream Points to find him.
Trapinch- Go to Rugged Mountain. He is there by default.
Ralts- Go to Spooky Manor. You need 10,000 points to find him.
You need those points to unlock their existence in the area, not to buy them like 2 Ralts for 20,000 Dream Points. 10,000 unlocks Bagon and Ralts (and other Pokemon that need 10,000 Dream Points) all at once in all the areas.