Roast the poblanos and jalapeño directly over a gas flame or under the broiler for 5 to 7 minutes, turning occasionally, until charred all over. Transfer the poblanos and jalapeños to a bowl, cover tightly with plastic wrap and let steam for 10 minutes. Peel and seed, then transfer to a blender. Add the tomatillos, cilantro, mint, garlic and lime juice, and process until smooth. Transfer to a large bowl. Drain the onions and stir into the bowl. Season the salsa with salt, if necessary. Serve.