Advancing through course AFTER clicking on some (but not all) buttons

Hi 

I have an activity where the user has 8 possible buttons to click on. They are asked to select three buttons of their choice. When they are clicked, their state changes to Selected.

They can then press a submit button to advance to the next page, where the next part of the activity will change depending on the choices they previously made.

I want to insert a rule on the initial page so they cannot advance if they have not chosen 3 of the buttons (or have chosen too many).

 

They only way I can think of doing this is through a series of complex triggers and lots of variables. For example, a true/false variable on each button, and then a separate numeric variable that adds/subtracts 1 when the true/false of each of the button changes. Then a trigger which only advances the slide when they click submit AND the numeric variable = 3. But this seems very long winded...

So - how do I instruct it to advance only if any three out of eight buttons have a state of Selected?

 

PS - I think the submit button is important so they have the chance to toggle the buttons from Normal/Selected and change their mind, otherwise I'd ditch the submit button and just use an automatic advance when a counter hit 3..

 

thanks in advance!