In a medium bowl, whisk together lemon juice, minced garlic, olive oil, salt, and pepper until well combined.

Add shrimp, scallops, and tuna to the bowl with the marinade, ensuring everything is well-coated. Cover and let marinate for 10-15 minutes.

Cut bell peppers and zucchini into bite-sized pieces. Alternate threading marinated seafood and vegetables onto skewers.

Preheat the grill to medium-high heat, around 375°F (190°C). Clean the grates and lightly oil them.

Grill skewers for 2-3 minutes per side until seafood is opaque and firm, with a char on the vegetables.

Remove skewers from grill, let them rest for a minute, and serve hot with lemon wedges.