The three Dreamers each have a general y position they will always spawn, those being top, middle and bottom. Their x position will be randomly picked from 2 or 3 options. Here is a graph of the exact locations, each color is a different dreamer.

Top (731, 339), (2111, 359), or (3467, 363) Middle (503, 1759) or (3547, 1395) Bottom (583, 3535), (1799, 3539), or (2867, 3459)

It's worth noting that they will wander in random directions and can stray away from their initial spawn. Knowing these spots, however, makes it easy to know where to find them or how to go for a quick route.