You express the ratio between 3 numbers by dividing down to their simplest form. Example: 100 to 200 to 300 would be divided by 100 giving the ratio between the numbers as 1 to 2 to 3. 7 to 21 to 28 would be 1 to 3 to 4. You divided each by 7 to reduce them to their simplest form showing the ratio between them.