Place the shrimp, red onion, jalapeno, cucumber, tomatoes, and cilantro in a large bowl.
In a smaller bowl, mix the lime, lemon and orange juices. Pour over the shrimp and vegetable mixture. Season with salt and pepper to taste and mix gently to combine.
Cover and refrigerate for 15 minutes. Add the chopped avocado and gently mix to combine. Serve.
Notes
Cooked shrimp keeps it easy and reliable: Since the shrimp is already cooked, the citrus marinade is mainly there for flavor, not food safety or texture transformation.
A mix of citrus gives better flavor: Lime gives sharpness, lemon adds brightness, and orange juice softens the acidity with a little natural sweetness.
Fresh citrus: Makes all the difference in taste. Skip the bottled stuff.
Small pieces make it scoopable: Chopping the shrimp and vegetables into small pieces makes this ceviche easy to eat with chips, crackers, or tostadas.
Adding avocado at the end keeps it fresh: Avocado is delicate and can get mushy if it sits too long in citrus, so fold it in right before serving.
Make-Ahead: You can combine the shrimp, vegetables, cilantro, citrus juice, salt, and pepper up to 8 hours ahead. Cover and refrigerate until ready to serve. Add the avocado right before serving.
Storage: Shrimp ceviche tastes best the day it is made, but leftovers can be stored in an airtight container in the refrigerator for up to 2 days. The vegetables will soften and the avocado may darken as it sits.
Authentic flavor with a shortcut: This Mexican-style shrimp ceviche is made with cooked shrimp, instead of raw shrimp for an easy, practical home version. It's fresh, bright, citrusy, and full of the classic flavors you expect from shrimp ceviche.