a number occurs 6 times and two other numbers occur three times each
Number coming 6 times can be chosen in $6$ ways and $2$ remaining numbers can be chosen in $\binom{5}{2} ways$
So total number of ways = $6 \times \binom{5}{2} = 60$ ways
Probability = $\Large \frac{60 \times \frac{12!}{6! 3! 3!}}{6^{12}}$