Steam shrimp and scallops, covered, 6 minutes or until done. Cool and cut shrimp and scallops into quarters. Steam mahimahi, covered, 8 minutes or until fish flakes easily with a fork. Cool and cut fish into 1-inch pieces. Combine the shrimp, scallops, fish, tomato, cilantro, shallots, lime juice, jalapeño pepper, and salt in a bowl. Spoon 1/4 cup seafood mixture into center of each lettuce leaf, and roll up.