Google Forms: Grouping multiple checkbox questions under one data validation

I have a google form with let's say, 3 checkbox questions, with about 5 options in each of them. I know that it is possible to make a data validation field (using the google form creator online) for each question to put a "up to n options selected" to limit the max options selected in each question.

But, instead of an individual data validation for each question, i would like to add a TOTAL max number of options across those questions to be enforced. How can this be done in the simplest possible manner with google app script, if not already possible with the Form creator?