Roast 2 of the whole peppers under a broiler or directly over a gas flame, rotating the peppers with tongs until completely blackened. Place them in a paper bag or wrap in paper towels to sweat for 5-10 minutes so that the peel is easy to remove.
Rub off the skin from the roasted peppers and remove the seeds. Slice and set aside.
Seed, core, and thinly slice the remaining red pepper.
Melt butter in a large saucepan. Saute the unroasted red pepper, onion, and garlic over low heat just until tender (about 10 minutes).
Add carrots and chicken stock. Bring to a boil, lower heat, and simmer until carrots are very tender (about 30-40 minutes).
Allow the soup to cool slightly before adding the roasted red peppers. Puree the mixture in a blender or food processor until smooth. Taste and season with salt and pepper, as needed.
This soup is delicious served warm (just return to the saucepan and reheat if desired), or you can serve it at room temperature on a hot day!