Put a grill pan over medium-high heat or preheat a gas or charcoal grill.
In a large bowl, drizzle the chicken, flank steak, and shrimp with olive oil and season on all sides with salt and pepper, to taste. Grill the chicken until cooked through, about 6 to 8 minutes per side. Grill the flank steak to desired doneness, about 5 minutes per side for medium. Transfer the chicken and steak to a cutting board and let the meat rest for 5 minutes before slicing. Grill the shrimp until just cooked through, about 3 minutes per side. Grill the scallions until brown, about 2 to 3 minutes per side.
Grill the cheese for 10 seconds on each side. Transfer the shrimp, scallions and cheese to a large plate.
In a large heavy saucepan, heat 2 tablespoons of the oil over medium-high heat. Add the onion, tomatoes, and garlic. Cook until the onion is golden and very soft, about 8 minutes. Add the chiles, oregano, and cumin. Stir until fragrant, about for 2 minutes. Stir in 1 cup water and bring to a boil. Reduce heat to medium-low and simmer until the chiles are soft, about 2 minutes. Let the mixture cool slightly, about 5 minutes. Transfer the mixture to a blender or food processor, add the remaining 1 cup of water and blend until smooth. Strain the sauce into a medium saucepan and simmer until slightly thickened, about 5 to 8 minutes. Season with salt and pepper, to taste.
Cut the chicken and flank steak into 3/4-inch-wide strips. Arrange the chicken strips, steak strips, shrimp, scallions, and cheese slices around the edges of a heatproof dish. Pour the warm sauce into the center. Serve very hot, with tortillas to make tacos.
The sauce can be made 1 day ahead, covered and refrigerated. Bring to a boil before using.