Only allowing use to only select 3 answers in multiple response quiz question

My client want to only allow the user to select 3 possible answers in a quiz question.

I figured I might be able to make it work by putting the submit button on a different layer, and have that layer appear when 3 answers are selected, and disappears again when more than 3 or less than 3 are selected.

I have tried different ways to make this work, including adding a variable that counts each time a answer is selected, but the problem with that is if the user de selects that answer, it still adds another one to the counter. I tried creating an action that would subtract the value to the variable but I am not able to get this to work. Can someone point me in the right direction, or does anyone have an easier way to make this happen.

I am using 360 and have attached a file with that quiz question.