Heat grill to high (450° to 550°). Rub corn and poblano with 1 tbsp. oil. Grill both, turning occasionally, until poblano is mostly blackened, 5 to 10 minutes, and some corn kernels have browned, 10 to 20 minutes. Let cool.
Working with one at a time, stand corncobs in a large bowl and slice down the sides; the bowl corrals flying kernels. Peel and seed poblano, cut into 1/2-in. pieces, and add to corn. In a small bowl, whisk remaining 2 tbsp. oil with the lime juice, chipotle chile, and salt.
Stir avocado, cilantro, and onion into corn mixture along with chipotle dressing.
Note: Nutritional analysis is per 1/2-cup serving.