Turn the oven on to the broil setting. Place salmon on metal baking sheet, drizzle 1 tablespoon olive oil over fish and sprinkle with ¼ teaspoon salt, and pepper. Broil 7-10 minutes or until fish is opaque in the center. Let cool completely and flake into chunks.
In a medium bowl whisk together lemon juice, remaining 2 tablespoons oil, chives, dill, parsley, and remaining ½ teaspoon salt.
Cut avocados in half lengthwise, reserving shells for serving. Remove pit and score ½” dice in the flesh. Scoop flesh into bowl with lemon dressing, leaving a ¼” thick shell. Add salmon and cucumber and stir gently to coat with dressing.
Scoop salmon mixture back into avocado shells and serve.