In a large serving bowl, combine the black-eyed peas, black beans, corn, chopped tomatoes, bell pepper, onion, cilantro and jalapeño. If you'll be including avocado, wait to dice it until you're ready to serve the dip, so it doesn't turn brown in the meantime.
In a cup, whisk together the dressing ingredients until emulsified. Drizzle the dressing over the serving bowl and toss until well mixed. Season to taste with additional salt and pepper.
For best flavour, let the mixture marinate for at least 20 minutes before serving. If you're adding avocado, mix it in just before serving.