Combine chicken, tomatillos, poblano peppers, Anaheim peppers, Serrano peppers, onion, garlic, cumin, and a big pinch of salt in the pressure cooker.
Heat over high heat until sizzling.
Seal pressure cooker, and bring to high pressure. Cook for 15 minutes. Release pressure.
Use tongs to remove the chicken thighs from the pressure cooker and set aside to let them cool.
Add cilantro and fish sauce to the contents of the pressure cooker.
Blend with a hand blender or in a standing blender and season to taste with salt (I don't add any).
Shred the chicken by hand. Remove the bones and skin and toss. Shred to a consistency that you want in the soup. It can be anything from big chunks to finely shredded.
Return the chicken to the sauce, stir, and let it sit for a few minutes so that the chicken absorbs the sauce.
Serve it however you like. Put it in a bowl and add a little cilantro, or sour cream, or shredded cheese. I like to dip tortilla chips in it.