Preheat oven to 450 F. Cut squash, peppers, and onion into bite-size pieces. Place in a large mixing bowl and coat well with Italian salad dressing (more or less to taste). Turn out onto a large cookie sheet or flat baking tray. Bake uncovered in preheated oven for 15-20 minutes until tender. Stir halfway through baking. When done, take out and set aside.