Halve one avocado, remove pit, and scoop flesh into medium bowl. Mash flesh lightly with onion, garlic, jalapeño, cilantro, salt, and cumin (if using) with tines of a fork until just combined. Halve and pit remaining two avocados, and prepare. Gently scoop out avocado into bowl with mashed avocado mixture. Sprinkle lime juice over diced avocado and mix entire contents of bowl lightly with fork until combined but still chunky. Adjust seasoning with salt, if necessary, and serve.