Heat the avocado oil in a large skillet over medium-high heat. Add onions and garlic. Cook for 1-2 minutes until fragrant.
Add the chopped zucchini, yellow squash, and carrots to the skillet. Stir the vegetables. Cook the vegetables for 8-10 minutes, stirring occasionally, until they are tender but still slightly crisp. Season the stir fry with salt and pepper to taste.