Make custom scoring on google forms checkboxes

i want to know what should i do in script editor especially CheckboxValidation custom scoring on Google Form :

I need some condition on e.g 1 question, the respondent can check 3 checkboxes available in this following condition :

** - Check all 3 boxes, given 2 points

• Check 2 boxes, given 1 point

• Check only 1 box, given 0 point **

On recently Google Forms version i create right now, it's only possible to given 2 points if we checked all 3 boxes, or else will given 0 point...

Any suggestion for the script i need to build matching with above?

*PS :http://ift.tt/2hnPu8p i've read all documentation on this but seems there's any method/function to do this kind of thing