In a medium bowl, combine the chillies, coriander, shallot, garlic, ginger, lime juice, kaffir lime leaves, soy sauce, vinegar and sugar. Pour half of the dressing into a small serving dish, reserve to dress the oysters. Add the olive oil to the remaining mixture.
Add the prawns and marinade for 30 minutes. If using wooden skewers for the prawns, soak in water for at least 30 minutes.
Prepare the barbecue for direct and indirect cooking over medium heat (190°C to 230°C).
Thread the prawns onto the skewers.
Place the crabs over direct medium high heat and barbecue for 3 to 4 minutes on each side. Add the prawn skewers to the barbecue and cook over direct medium heat for 2 minutes on each side. Add the oyster to the barbecue and cook over indirect medium heat for 4 minutes.
Remove all the seafood from the barbecue and serve onto a platter. Finally, spoon the reserved Thai dressing over the oysters.