Rinse and pat dry the fish, shrimp and scallops. Cut all in very small pieces (about 1/4 inch). Place in a glass bowl. Cover with lime juice and 1/2 tsp salt. Make sure the fish is completely covered, if needed add more lime juice.
Cover and refrigerate for 4 hours, stirring occasionally.
Add jalapeno and onion and refrigerate for another 2 hours.
When ready to serve, add tomatoes, avocado, cilantro and 1/2 tsp salt.
Garnish with lime wedges.
You can make variations of this dish by omitting the scallops, shrimp, or using just one type of seafood. I personally like the combination of the three which makes it more interesting.