If using uncooked shrimp and calamari, bring a pot of salted water to a boil. Add the shrimp and cook for 2-3 minutes until pink, then add the calamari and cook for an additional 1-2 minutes. Drain and let cool.
In a large mixing bowl, combine the cooled shrimp, calamari, and crab meat.
Add the diced cucumber, halved cherry tomatoes, sliced red onion, and chopped parsley to the seafood mix.
In a small bowl, whisk together the olive oil, lemon juice, Dijon mustard, salt, and pepper to create the dressing.
Pour the dressing over the seafood and vegetable mixture. Gently toss to combine until everything is well coated.
Taste and adjust seasoning if necessary, adding more salt, pepper, or lemon juice to your liking.
Serve immediately, or refrigerate for up to 30 minutes to allow flavors to meld before serving.