Script Runner - Customer Field - to take the value of other custom fields

[build-automation-bot]

Hi, New to Script Runner and looking to see if I can set-up this custom field

Here is the scenario

Field A can be  either  - 1,2 or 3

Field B can be  either  - 1,2 or 3

Field C can be  either  - 1,2 or 3

Need some logic to automatically make Field D the highest value of these 3 fields e..g if Field C is 3 and A & B is 2 Make Field D = 3. In the same way if Field A = 2 and Field B & C = 1 then make Field D = 2 (the highest value).