Clean crabs, removing gills and sandbag. Cut into halves.
Steam or poach crabs until meat is cooked and firm. Cool and shell. (Reserve a few claws unshelled for garnish.)
Steam or poach prawns and squid. Cool seafood and combine in a bowl with the rest of the salad ingredients. (Salad can be made up to this stage several hours ahead. Cover bowl with clingfilm and refrigerate.)
Stir the dressing ingredients together in a bowl and pour over salad.
Toss, taste and adjust seasonings if necessary and serve immediately.