Place green beans in a covered pot with a scant 2 inches of water, and cook over medium heat until beans are tender-crisp, about 10 minutes. Add peas; cook slowly for 5 minutes. Add potatoes and cook about 8 minutes more, or until potatoes are just done. By this time almost all water should be cooked away; add extra only if necessary to prevent burning. When vegetables are cooked, add regular or evaporated milk just to cover, butter, and salt and pepper to taste. Heat thoroughly but don’t boil.