Finely slice fish. Place fish, prawns and scallops in a colander and rinse well under cold running water. Pat the fish dry with paper towels.
Grill the prawns and scallops, then slice in half lengthwise. Place all seafood in a container. Add remaining ingredients and mix well. Refrigerate for a minimum of 3 to 4 hours before serving.
To make the relish, mix avocados cucumber, cilantro and lemon juice in a non-reactive bowl. Season with salt and pepper, to taste. Chill prior to serving.
To plate, place ceviche on a bed of lettuce, top with relish and garnish with cilantro.