Galarian Moltres appears flying around the isle of armor. It seems to go in a circle around Soothing Wetlands, the sea behind the Tower of Waters, the station, and the dojo. Intercept its path and whistle at it to battle it.
Galarian Zapdos appears in the Wild Area. It runs around the part of the wild area you can access early game at high speeds. Use your ability to bike on water to get in front of it and bike in to it to battle it.
Galarian Articuno appears in the crowned tundra. When you find it, it will split in to three, and you will have to encounter the real one. If you guess the real one correctly, it will teleport to you and attack you.