About multiple answer in quiz

A short step-by-step description:

1. Define a user variable for each of the answers.
2. Use the checkbox interaction to create the custom question slide and link the user variables to the check boxes
3. Create two Next buttons, hidden in output, can be in same location if they are not in a normal fluid box. They should be identical, but the NextCorrect button has a score of 1 point attachted. The NextWrong button has no score attached.
4. Create a custom Submit button which will trigger an advanced action to check the  chosed checkboxes based on the values of the user variables. That may be the tricky part, you show the NextCorrect button if at least 3 answers are correct, the other one if that is not the case. I would use a counter in this case. You can hide the Submit button, and have the chosen Next button in the same location. It should be the only way to navigate to the next slide. 

I have described that dual button workflow many years ago (has been stolen by other experts without paying credit). This is the original post, but was still for SWF output:
https://blog.lilybiri.com/report-custom-questions-part-2

This will only be possibel if you create a custom question slide, just an example by using the Checkbox Learning interaction, and by creating an advanced action which checks the givne answers. 

How do you want to score the question? Always same score if at least 3 correct answers are given, but allowing them to give even 4 or 5 correct answers? Or a more complicated scoring?

I don't see how partial scoring can solve this at all, but may understand your question not completely.

https://medium.com/@hdecisions/building-a-fully-custom-quiz-in-adobe-captivate-using-javascript-4a12a33a57b4

There is no method to score the answer correct for only three our the five correct answers. However, you can use partial scoring and perhaps penalties to still make this question work similar to what you want.

1. Select Multiple Answers in the Quiz panel
2. Select Partial Score in the Quiz panel
3. One by one select each correct answer and click on Properties and then the Options tab
4. Assign a points value for that one answer
5. Optionally you can select the incorrect answers and assign a penalty so if those answers are selected as part of the answer they will lose a certain number of points.

So this way if a learner selects all five correct answers they will get full marks. If they select fewer than the five they will still get a partial score and if they select every answer it will still be partially correct but they will lose points for selected a distractor as well. Here is a video tutorial I did on this subject if you would like to see an example.

https://youtu.be/Ib5U54xToEU