Preheat the oven to 375 degrees.
Put the tomatillos, 2 of the apples, the onion, garlic, and jalapeño on a rimmed baking sheet. Toss with the olive oil and season with salt and pepper. Roast until tomatillos are softened and slightly charred, about 20 minutes.
Peel the garlic, then transfer all of the ingredients to a blender and puree until smooth. Season with salt and pepper.
Chop the remaining apple into 1/4-inch cubes and stir into the salsa before serving.