Bring a large pot of water to a boil. Add the rice noodles and cook according to package instructions (usually 7-10 minutes). Drain and rinse under cold water to prevent sticking. Set aside.
While the noodles are cooking, slice the bell peppers, cut the broccoli into florets, and julienne the carrots.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add minced garlic and sauté for about 30 seconds until fragrant.
Add the sliced bell peppers, broccoli, and carrots to the skillet. Stir-fry for about 5 minutes until the vegetables are tender but still crisp.
Mix soy sauce, oyster sauce (if using), sesame oil, sugar, salt, and pepper in a small bowl.
Add the cooked rice noodles to the skillet with the vegetables. Pour the sauce over the noodles and toss everything together well, ensuring the noodles are evenly coated. Stir-fry for an additional 2-3 minutes.
Remove from heat, garnish with sliced green onions, and serve hot.