There are a few combinations of mixed numbers that can have a sum of 3. A mixed number must have one whole number and one fraction. Combinations of mixed numbers that can have a sum of 3 are 1 and 1/2 + 1 and 1/2, 1 and 1/3 + 1 and 2/3, 1 and 5/8 + 1 and 3/8 and so on.