Hi,
I think that a quick and dirty trick would be to change the points for each question: add 1 arbitrary point to each of the three answers options.
The student must be advised that the score shown when the quiz is finished is a raw score and the real score will be calculated/shown shortly after that:
Each question will now have:
correct= 3 points and incorrect= -1 point. Ommited answers will score no points (default Moodle behaviour).
When a quiz is finished, you will need to substract n number of points, one point for each question, You will then have a score that actually added 2 (3-1) points for each correct answer, deducted 2 (-1-1) points for each incorrect answer and deducted one point (0-1) for each missing answer.
The corrected score is very easily calculated (= quiz_score minus number_of_questions_in_quiz) and:
Friendly mode: the student could not see the raw quiz score right after finishing the quiz, but the student can later see a conditional resource that shows the adjusted score (a few seconds) only after the quiz has been submitted and marked.
Quicker mode: the student is instructed to deduct the number of questions inside the quiz from the raw quiz score recently shown.
Issue solved