Combine orange juice, lime juice, lemon juice, vinegar, olive oil, garlic, cumin, pepper flakes, coriander, salt, and pepper in a blender or food processor. Pulse several times until mixture is well blended and smooth.

Place chicken breasts in the bottom of a slow cooker. Pour juice mixture over all, cover, and cook on High setting until no longer pink in the center and juices run clear, about 2 ½ hours. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).

Shred cooked chicken with two forks, adding a little liquid from the slow cooker, if desired. If serving as a main dish, remove chicken to a plate, cover, and keep warm.

Strain liquid into a 1-quart saucepan and bring to a boil over medium-high heat. Simmer until liquid is reduced to 1 cup, about 10 minutes. Combine cornstarch and water in a small bowl and stir until there are no lumps. Add cornstarch mixture to the saucepan, stirring continuously. Allow mixture to boil about 2 minutes, stirring continuously.