Preheat oven to 400°F; line large sheet pan with parchment or foil.
Arrange potatoes and broccoli on pan; drizzle with 2 tablespoons oil and season with half the garlic powder, Italian seasoning, salt, and pepper.
Push vegetables to sides; place salmon in center, drizzle with remaining 1 tablespoon oil, and season with remaining spices.
Top each fillet with lemon slice; arrange remaining slices around vegetables.
Bake 18-22 minutes until salmon flakes easily with fork and vegetables are tender and lightly browned.