Place the tuna, prawns, and salmon each in a large bowl and add 1/3 of the avocado, lime juice, and coriander to each bowl. To the tuna, add ½ of the ginger, and all of the yellow chilli pepper, sesame seeds, sesame oil, and spring onion tops. To the prawns, add ½ each of the red onion, finely chopped red pepper, olive oil, ginger, and garlic.
To the salmon, add the remaining red onion, finely chopped red pepper, olive oil, and garlic, and all of the red sweet pepper and sweetcorn. Season each with salt and pepper, and marinate for 10 minutes.
Serve the 3 ceviches together, each on a large decorative spoon or in a serving bowl.