In a bowl, combine chicken and 1 teaspoon of cornstarch until the chicken is fully coated. Marinate for about 5 minutes.
In a bowl, combine shrimp and the remaining 1 teaspoon of cornstarch until shrimp is fully coated. Marinate for about 5 minutes.
In a wide pan or wok over medium-high heat, heat 1 tablespoon of oil. Add chicken and cook until color changes. Remove from pan and keep warm.
In the pan, add another tablespoon of oil if needed. Add shrimp and cook just until color changes. Remove from pan and keep warm.
In the pan, add sausage and cook, stirring regularly, for about 1 to 2 minutes until lightly browned. Remove from heat and keep warm.
In the pan, add another tablespoon of oil if needed. Add onions and garlic and cook until softened.
Add broccoli and cook, stirring regularly, for about 2 to 3 minutes or until almost done.
Add chayote and cook, stirring regularly, for about 2 to 3 minutes or until almost done..
Add carrots and cabbage and cook, stirring regularly, for about 1 minute or until almost done.
Add noodles, stirring gently to loosen and evenly distribute.
Return chicken, shrimp, and sausage to the pan.
Add water or broth, oyster sauce, and soy sauce.
Gently toss and stir, adding more liquid as needed, until noodles are done, chicken, and shrimp are cooked through, vegetables are tender-crisp, and liquid is mostly absorbed.
Season with salt and pepper to taste.
Transfer to a serving platter and garnish with chopped green onions and fried garlic bits. Serve with calamansi wedges, if desired.