1 part to 3 parts means that for every 1 part of an ingredient added, 3 parts of another are added. A vinaigrette salad dressing is a good example. Let’s say a recipe calls for 3 parts oil, one part vinegar. That means for every measure of vinegar added, 3 equal measures of oil must be added.
Let’s say a salad dressing recipe calls for 2 parts oil and 1 part vinegar. The ratio of those two ingredients is 2 to 1. That means that the amount of oil will be double the amount of vinegar. If you would like a small amount of dressing, say 3 ounces, you would use 2 tablespoons oil and 1 tablespoon vinegar.