To make the seviche: Toss the shrimp with the lemon, lime and orange juices in a large non-reactive bowl. Refrigerate for a minimum of 3 hours or until the shrimp have turned opaque.
While the shrimp is "cooking," make the Xni Pec: Toss the tomato, cilantro, olive oil, onion, chile, and lime juice in a bowl to mix. Season with salt and pepper, to taste.
When the shrimp are ready, drain them and discard the marinade. Toss with the Xni Pec. Divide among martini glasses or other serving dishes and garnish each with a corn tortilla chip. Pass remaining chips separately.
Peeled and deveined shrimp are now available at most markets. You will save a considerable amount of prep time by purchasing "P and D" shrimp instead of those that need to be peeled and deveined. Choose your chile depending on how much heat you can tolerate. I love heat, so I would go with a habanero chile seeds and all, about the hottest there is, for this. For a milder dish, choose serranos or jalapenos and remove the seeds before chopping.
Recipe courtesy of Daisy Martinez, 2008