Heat a grill to high (450° to 550°). Brush chiles, corn, and onion with 1/2 tbsp. oil. Grill, turning as needed, until grill marks appear and onion is somewhat softened, 8 to 10 minutes. Let cool.
Peel, stem, and seed chiles and cut into large chunks. Cut corn kernels from cobs. Coarsely chop onion.
In a bowl, toss vegetables with lime juice, remaining 2 tbsp. oil, and salt and pepper to taste. To serve, stir in tomatoes, cilantro, and chives.
MAKE AHEAD Through step 3, chilled airtight up to 2 days.