This was an interesting one!
You can actually just logic why the solution cannot be 36. Since the last number (smallest) is added, the product of the first two can be no smaller than 30 and no larger than 35 (last die cannot be greater than 6 or less than 1). Factoring the numbers between 30 & 35 leads to only one number that has 2 paired factors between 1 & 6: 30, with a factor pair of 5 & 6. Given the constraint that the added number must be less than or equal to the lower number the maximum result with a product of 30 is 35 (6*5+5).