The question is not really clear..
you want to make 3 buttons where one is showing the correct answer..
button one set answer to : incorrect answer
button two set answer to : correct answer
button three set answer to : incorrect answer
So that would be the easiest way..
If you want to make it so Construct2 makes random questions with random answers, you should create the maths for that yourself..
Adding or subtracting something from the correct answer will make it incorrect, so that shouldn't be so hard..