You will need to create a random number in an equation question (hidden) and then show and hide the questions 5-7 via conditions.
Have a look in the forum or in the manual, it should be described how to create the random number. I just had a discussion about this this week, so if you click on my name and look at the topics, you will find a discussion on how to do this.
Had a quick look and the thread is here, but unfortunately in German. So if you are not able to read german you might have some problems.
Code:
{if(randnumber>0, randnumber, rand(5,7))}
This is what you need to write into the equation question. Then it should give you a random number which is either 5, 6 or 7.
Based on this you can create conditions that are shown/hidden via conditions, based on the equation question.