Adding (sum) of two button sets to trigger next button

I am creating a course which mirrors a personality type assessment.  Each slide has 2 button sets with values for each button (1, 2, 3).  I want to disable the next button until the sum of the two button sets is exactly 3.  

So, the user can put in any combination of selections from button set one and two but, they can not exceed or fall short of the sum of 3.  

I have placed triggers for their number values to reflect on a different counting variable but, I want to have a variable and trigger that adds the two sets and allows the user to move to the next question only when the sum of these sets is equal to 3.

Any help?

Thanks in advance

 

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